Since I am not to keen on licking ballpoint pens; if you have shoes on with rubber soles, draw a few lines on the bottom of your shoe...
After that, go to the paper and it will almost instantly write again.
Since the ball in the pen get clogged up, the rubber pushes the ball loose from the clogged up ink.
The drawing of the lines makes sure the small cloggs are transferred to your sole instead of your tongue.
lifehacks.stackexchange.com/users/128/RichardBernards
Lick them vigorously.
Ideally, just the point - but if you get too enthusiastic, no harm done.
Ballpoint pens work by using a small rotating metal ball to transfer ink from the internal reservoir to the paper, and over time ink can gum up the ball preventing it from rolling.
All you need to do is soften the ink and your pen will work again!
If the ink in the reservoir has dried, you'll have to replace it entirely though.
lifehacks.stackexchange.com/users/87/William the Pleaser
Heat will help the ink flow.
hold the tip of the pen under running warm water, or place the pen in sunlight for some time, or hold the tip of the pen near a candle flame for 2-3 seconds and quickly remove it from the flame.
After that, scribble on a piece of paper, it will start working.
lifehacks.stackexchange.com/users/None/
The idea is that the tip is clogged.
And you want to allow it to flow again.
--
You may try to dilate the tip (and slim the ink) with heat to allow the ink to flow.
Other answers have already pointed to heat based solutions, such as using a candle.
Another option for the heat idea is to use pen against glass very fast.
Glass won't get a permanent mark, and it will be hard for the ballpoint pen to scratch it.
The fast motion is meant to heat the point of the pen by friction.
You may also try to use gravity (leaving the pen in a verticall position) to try to force the ink to flow.
Other options are to use centrifugal force (that is a virtual force, it exists), or - in the case of cheap rollerball pens - dismantle the pen and blow the ink to the tip from the opposite side - beware of excess flow, do it against something that can absorb the ink.
On the other hand the ink may be dry.
If it is not totally dry, you may try to mix it.
What kind of ink is it?
Ink is usally a suspension in water, oil or alcohol.
Rollerball pen usually are water based, others may use oil.
Alcohol is only common in markers.
Whatever it is... add that to the ink - or the tip, if you don't have access to the ink.
Water (Saliva from licking has been also suggested), Mineral Oil (Johnson's Baby Oil will do) or Alcohol (Isopropyl or in its defect, Methanol), depending the case.
lifehacks.stackexchange.com/users/11574/Theraot
Usually shaking the pens or leaving them upside down will do the trick.
I also have a few pens that I really like, but I'm having problems with the ink
so I find a working pen with the same or similar ink reservoir
and I will switch the two.
lifehacks.stackexchange.com/users/20/michaelpri
There's one solution /
it works all the time!
Wet a napkin and take the ink cartridge and roll it up in the napkin/ place in mic for 20 sec !
lifehacks.stackexchange.com/users/12501/Generro g
Seconding user19 's point about heat , but if neither running water, sunlight or flame is available try the following:
For most cheap ballpoint pens, the bit that actually holds the ink is a narrow piece of plastic tubing with a nib on the end.
You can usually remove this part from the outer casing by taking off the end cap at the back of the pen and tipping it out.
Gently flex the tube with your fingers a few times.
The motion plus the heat from your fingers frees up the ink flow.
This is assuming that the pen is broadly similar to the "Bic" brand.
I've included that link for illustrative purposes only and not at all because some of the reviews are hilarious.
lifehacks.stackexchange.com/users/10729/Lostinfrance
Try shaking your pen, placing gas at the top of the pen with a lighter for at max 7 seconds minimum 3 seconds as the ink may turn red or the plastic will burn.
Or how about rubbing your pen at the bottom sole of your shoe, that often does not work for me though.
The easiest way for me is to draw plenty of swirls on a piece of paper lightly to begin with then with more pressure as it help build up the surface area strength.
You can also just buy a new pen.
lifehacks.stackexchange.com/users/10966/homebound
Actually I had these great pens which I seriously loved and used them
but after around the first or second time I used them they stopped working.
So I saw this post and tried various methods but what worked for me is that I removed the refill and separated the metal nib part from the plastic part and then put a very tiny amount of nail polish remover into both parts
then I shook them for a minute and rubbed the big forcefully on to an eraser because the ball wasn't moving
so I did this to the other pen too and it worked.
Also keep some nail polish remover handy in
case you got some ink on your hands.:)
lifehacks.stackexchange.com/users/11627/Ayukta
I found a box of several old ballpoint pens. When I open them up, I can see each has a significant amount of ink in it. However, when I try to write with one, no ink is transferred to the paper.  I've tried running them under water, but this doesn't work very often. (< 25%) Is there any way to revive the pens?
How to revive dried up pens
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lifehacks.stackexchange.com/users/15/Mooseman
There are many different ways to solve this problem. You could draw some lines on your shoes or lick the nib of the pen. Alternatively, you could consider shaking the pen or applying heat to it, such as warm tap water. Solutions include running the pen over rubber shoe soles, licking vigourously, holding under warm water or over a lighter for a few seconds, heating with a candle, shaking the pen or leaving it upside down or wrapping in a wet napkin and microwaving for 20 seconds. If the ink has gone dry you will have to replace the missing solvent.
Run the pen over the rubber of a rubber soled shoe. Lick vigorously to loosen the ink. Heat up under warm running water for 2-3 second to loosen the ink. Heat the tip using for example a candle or run it quickly over glass and if the ink has gone dry replenish it with the appropriate solvent. Shaking the pen or leaving it upside down usually works. Roll up in wet napkin and microwave for 20 seconds. Remove the nib and flex the tube between your fingers to loosen the ink. Warm the pen with a light for 3 - 7 seconds or swirl over paper pressing down with different amounts of force.