text: Q:
  
  What to do when I accidentally kill the whole party?
  
  So, my players' PCs were fighting 11 fifth-level orcs. They are five 5th-level characters in D&D 4e. It ended in an unexpected TPK.
  Thing is, their bard was new (he was put in place of their cleric), with not enough magic items to be as strong as the others and, as everybody knows, a bard is not as powerful as a cleric.
  It was a damn hard combat, though my players were used to it and they used to play very well together, making some combats easy even when they were meant to be overwhelming. This one was supposed to be quite hard, but not overwhelming.
  What happened was: the wizard miscalculated his Color Spray, hitting the fighter, who lost his Rain of Steel stance. They didn't deal with most of the orcs during the crucial time they had with many dazed enemies, because of the Wizard's mistake. It was brutal. They didn't get a break after that for the whole combat and finally, were obliterated by the orcs.
  Now I feel like I killed 5 characters, but only one had it coming. The other players were disappointed because they all feel like it was the wizard's mistake that killed them all (and I also believe it, BTW). And the one who plays the wizard was the only one who was "Yay! Finally I get to play my hybrid wizard/warlord!" which pissed me off a little bit more, I've gotta say.
  They are all making new characters but they feel like the campaign ended prematurely. They also don't wanna go back to level 1, but for that matter, I don't feel like GMing for brand new characters that already start as strong ones.
  So, how to deal with this situation?
  
  A:
  
  It sounds like nobody was happy with the outcome of this game session.  The point of gaming is, on its most basic level, to have fun together as a group.  You aren't having fun.  So, you need to change the outcome.
  There are a few approaches available.
  Ret-Con
  This just didn't happen.  Maybe you want to "rewind" the whole encounter to a certain point and keep playing.  Or, you could choose to re-write parts of the events and outcome such that the characters (except, perhaps the wizard who's happy to die and might have had it coming) managed to get away.
  Deus Ex Machina
  The events of the past haven't change (much), but some amazing Plot Device comes along right after the fight and saves/resurrects the PCs.  Again, the magic might run out before the wizard's turn, if you so choose.  You can incorporate this into a new plot thread, with the thing/person which saved the PCs being a new ally, a side-effect of strange phenomena, or even a villain looking for recruits.
  Some possible ideas:
  
  The characters are actually just unconscious.  They can wake on their own, or be revived by a helpful NPC.
  The characters are dead, but appear together in an underworld.  The story continues with a chance at resurrection (see comments from Iwillnotexist)
  A curious magic in the area brings them back, and now it's time to investigate
  
  Make new characters and start over?
  Yeah, you can.  If your group really enjoys the gamist aspects of D&D encounters and loaths "breaking the rules", you may decide to live with the results.  Changing this event after the fact can reduce the sense of risk for the future, diluting the experience.  
  If you do want to keep the results and start over with new characters, you may consider starting with a higher level.  Maybe go down to 3rd or 4th, so there is a real loss, but you aren't stuck all the way at the start.
  However, your dissatisfaction suggests that keeping this outcome isn't satisfying.  I don't like do-overs in my stories if I can avoid them (in almost 15 years, I've seldom changed a major event), but in extreme cases it's worth it.  Just don't reach for this kind of tool unless there aren't good alternatives.
  Involve Your Players
  The problem you're trying to solve is a problem for your whole group.  In order to solve for everyone, you should get their input on the results.  Present the basic options above and discuss how people feel in order choose something that everyone will be happy with.
  The GM's Part
  As the GM, you're still in the driver's seat here, even with your player's collaboration and input.  It's up to you to guide the discussion, hear people out, and end it when you've heard enough to make a final call.  
  You can still keep some amount of mystery and control over the story.  If the players want to rewind, you get to prepare new material to replace the old events.  Add in a surprise that will make the new encounter feel fresh and important, instead of a retread.  Be extra careful about balancing things and adding in contingencies so that the party feels challenged but isn't going to die again.
  If the players want a Deus Ex Machina, you've got lots of creative freedom on what the plot device will be and how it will play out.
  
  A:
  
  As already discussed its far easier and subtler to reduce things before you reach the point of a TPK. For example some of the orks could have been weaker than others, let them die after just one or two hits. If the players have any friends or allies in the area then they could ride to the rescue (or indeed new friends could turn up, everyone hates orcs).
  However it's too late now, so you have three main choices. 
  
  Rewind time. Personally I don't like this option and I know my gaming group would hate it. If you do that then actions have no consequences, failing can never happen. If you can't fail then it takes all the risk and agency out of playing. It might work for some groups but it will not work for many others.
  Roll new characters, maybe let them carry over some xp or levels.
  A more minor retcon: 
  
  Give each of them a chance to roll to stabalize. (For example make two out of three fort saves). Each of them that makes the roll survived and woke up in the dirt surrounded by the bodies of those who failed and with all their magic items stolen. They can now go hunt down the orcs to avenge their fallen friends and reclaim their missing items.
  As the previous bullet point, but instead of being left for dead the orcs took them prisoner and they need to make an epic escape.
  
  Another option would be to do something like tell all the players that if they want to generate a new character they can sacrifice their old one (i.e. take an automatic failure) to give all the others a +1 on their rolls. This lets the wizard player who wants to generate another character anyway make it up to the others if he chooses by having the wizard die and give the others a better chance.
  Maybe discuss that option with him ahead of time to see if he's interested before offering it to the group though. If the offer is there and he refuses that might make things worse not better.
  
  A:
  
  A side quest to return (i.e. "Return from Hell")
  Neither a retcon, nor a deus ex machina: you can have the party dropped into a new afterlife world. From there, they (or their souls, mechanically the same thing) must find their way out. This is satisfying because they can resolve the situation through playing forward, rather than returning to something in the past.
  This trick was done to good effect by "DM to the stars" Chris Perkins in the PAX 2010 Penny Arcade DND Celebrity Game (Aquisitions Inc Season 5).
  
  