Key not working when you hit it? There's crud under the keys. You need to clean it. It usually happens to the key you use most, usually the enter key or space bar.

Cleaning it turns out to be stupid easy.

Take a toothpick. Run it along the groove between the rows of keys. You'll probably find a bit of fluff when you drag it up at the end. If you look very carefully you might even see a clump of fluff under the sticky keys.

Keep running the toothpick along vertical and horizontal rows until you don't get any more clumps of fluff come up. I just did it to a 6 year old Thinkpad that had never had this and got enough fluff to make a small kitten.