I have no idea what happened but in the last few days I've noticed that my delete key doesn't work and nothing happens when I press it. How can I fix this?
Are you sure the key isn't just broken?
Try booting to your Mac OS X install DVD (insert the disc, then hold the Option key on startup). See if the delete key works in that environment. If it doesn't, then you've eliminated pretty much all software problems and the most likely explanation is that the key is broken.
Depending on what kind of keyboard it is (part of a laptop?), you might be able to try to open it up and clean any grime that might be preventing good electrical contact.
If the keyboard is covered under warranty or AppleCare, just take it into an Apple store and they'll take care of you. Otherwise, you might need to just get a new one.