Most cars made in the last ten or fifteen years can be unlocked with a keyless remote, and once you get used to them, unlocking your car with a traditional key feels like a pain. The most common reason they stop working is just because the battery needs replacing, but when they're broken for real, a new one can cost a small fortune. So before you go out and buy one, try this quick fix that worked for Redditor diggalator, and cost absolutely nothing.
Basically, he pried the remote apart and took out the board, noticing that the 'Lock' and 'Unlock' buttons had come loose. All it took to fix was a few tiny drops of solder (and a soldering iron, of course), and when he put it back together, everything worked perfectly again. Talk about simple!
The best thing about this fix is that it takes less than half an hour and can save you a bunch of money. Plus, if you have the equipment, it doesn't cost a thing.
You can find more details and photos here.