Whether your brand new bb gun won't work or your smoke alarm wont shut up, you can fix (mostly) anything.
Step 1: Find the Problem
Try to find if something is lodged in where it shouldn't be or if something wont move and it should ( WD40 ) or if something is moving and it should ( Duct Tape. ) If the problem is electrical try to look for faulty or loose wiring. Remember when dealing with wires, be careful. I know this sounds childish, but try to fiddle with it. Maybe, it will start to work again.
Step 2: (Optional) a Second Pair
If you have another one of whatever you have that is broken, and don't want to replace it, compare the two. Look for wires that are attached differently, or stuff that shouldn't be there and is, or stuff that is supposed to be there but isn't. ( In this case simply replace what you don't have. )
Step 3: RTFM
Get the manual! It may sound basic, but it never hurts to refer to the manual to see if they have a problem section, and maybe some solutions.
Step 4: A Second Opinion
If you have someone you can ask. Ask! Somebody else may see something that you don't. It is always good to get another opinion, or another set of eyes.
Step 5: If All Else Fails
If you can't fix it, your friend, Father, Mom, Brother, Sister, Children, relatives might, ask for help. It won't blow your manliness if you ask for some help. If no-one you know can fix it, and you can't, and if you are to big to go to a repair store, or send it back for another, you can destroy it. If all else fails, and there is no hope in fixing it. You might as well destroy it ( or strip it for parts... )