Its possible that ...
a. Your wifi router was hacked. If you didn't use what protection is available, a driveby user could reprogram your wifi router as they see fit.
Fix? Reset it and set all the settings again.
b. The wifi router has failed. These products don't last forever. Try the reset button and then set it up again. If it fails, then try another router.
I have been using my Microsoft MN550 USB Wireless Adaptor for quite some time without any problem. All of a sudden, sometime last week I was not able to log on to the internet. I got a message saying the WEP security key does not match that of my router (an SMC Wireless Router). I do not use WEP key on my router. Nor do I use WEP key on my adaptor.( I know I should!!!). Where does this 'mismatch' come from?? The status of the wireless connection indicates the connection is there and the signal is 'very good'. Where can I go to fix this problem? Thanks.