Can you connect using your ethernet or USB lead?
If so then ensure that the wireless setting all match up. It's possible that your router has reset all setting so that if you have changed any WEP keys they may have been set to default.
You may also find that your admin username/password has been reset to the default.
My internet access is limited at the moment and am only able login every few days so if that doesn't work I'm sure somebody else can help you.
It would also help if you could tell us what model your router is.