You checked the connection, but did you disconnect the keyboard and then reconnect it?
Is it USB (flat rectangle at the end) or PS/2 (round at the end)?
Can you borrow another keyboard to try out on your computer?
Also, can you try your keyboard on another computer? If so, does it still have problems?
The problem could simply be that your keyboard doesn't work anymore. Thankfully, if that's the case, they're pretty cheap and easy to replace these days.