If you have 5-10 year old sons, daughter, nieces and nephews (basically kids you trust with a ring), get a group of them together and tell them that whoever finds the ring in the grass will get $20 (or some other appropriate, parent-approved prize).
Also have cake & ice cream for those who didn't find it.
lifehacks.stackexchange.com/users/4198/BrettFromLA
Go outside that night with a very strong flashlight or preferably a higher-intensity floodlight.
With any luck, the glint of any metal object will stand out considerably compared to anything else you might find among the grass.
Work systematically.
Search the lawn one square at a time.
You are much more likely to find something searching one square meter at a time rather than wandering aimlessly.
lifehacks.stackexchange.com/users/80/Robert Cartaino
Obvious but expensive answer: wave a metal detector over your lawn.
Maybe a friend has one you could borrow?
lifehacks.stackexchange.com/users/4198/BrettFromLA
First of all, do not assume it is in the garden unless you are absolutely certain.
If there is any possibility it is somewhere else, then those other places must be systematically searched.
Do not use a rake.
Do not walk around on, or otherwise disturb in any way the search area.
Randomly casting around for the object is a bad idea.
The way to find the ring is to systematically search the search area.
To do this with a plot of ground, like a backyard, what you should do is string it with a grid.
Drive in wooden stakes around the perimeter of the area.
Make sure the stakes are well outside anywhere the ring could have dropped.
Next, pull twine around the stakes making a lattice pattern.
Tie a heavy object to the twine and throw it across
so you do not have to walk in the search area.
When you are done you should have the yard divided into a grid, each cell being about 16 inches square.
Once you have divided the search area into a grid, begin searching it cell by cell, marking each cell after you finish it.
If your ring is in the search area, it is likely you will find it as long as you thoroughly search each cell.
Also, check all branches and twigs in each cell; sometimes an object will catch on a twig.
lifehacks.stackexchange.com/users/5292/Tyler Durden
Crows here in the Pacific Northwest find everything.
I threw a loaf of bread squares onto my lawn one morning, trying to remember where I put my single car key.
Voila, a crow buggered around and flipped the key so I could see it.
Luckily he didn't use it for his nest.
lifehacks.stackexchange.com/users/5299/Angus Blackwylde
You could try raking the grass, with any luck the ring might hook onto one of the many tines on the rake and thus be retrieved.
If it doesn't hook on, it should be easy to hear the metal on metal 'ding' noise once you've scraped over it.
lifehacks.stackexchange.com/users/2089/MrPhooky
You could also try the old vacuum panty hose trick.
Your neighbors might think you are crazy, if they see you vacuuming the lawn :)
lifehacks.stackexchange.com/users/2340/RobAu
How do I find my gold ring which is lost inside the grass in my garden?

Please tell me an easy way to find my ring quickly.
Finding a gold ring lost in the lawn
lifehacks.stackexchange.com
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lifehacks.stackexchange.com/questions/6712
lifehacks.stackexchange.com/users/4505/Vasanth Kumar
There are a few ways you can find a gold ring lost on the lawn. If you have young children, you could offer a reward for whoever finds it. Otherwise, you could try going out at night with a strong flashlight to catch any reflections, and work systematically, one square at a time. If you know someone with a metal detector, this could also be very useful. The options include getting a group of children to search, look at night with a flashlight, use a metal detector, make a string grid and go over it systematically or try the old vacuum panty hose trick.
Round up some children and offer a suitable reward. Search at night with a flashlight and look for the glint. Use a metal detector. Make a grid on the lawn with string and search each square systematically. Put some bread down and watch the birds - one may pick up the key. Use a rake - it may capture or clink against the ring. Try the old vacuum pany hose trick.