Airport extreme card - reset?

I'm having problems with my airport extreme on my imac (OSX 10.4). It suddenly started asking for my password, but it doesn't seem to like the one I put in. I just can't seem to get it to work, so I've tried other passwords - to no avail. SO, can I reset the airport card? If I go to keychains and ask it to show the password, it won't without me putting in a password..... What is happening? I'm not a computer expert - turn on the computer and off I go is my normal state.... Not used to having any problems at all with the mac. Other computers can log into the wireless router (all pc s) just not the mac.