My first instinct would be a driver issue, have you tried reinstalling it? Another routine procedure would be a firmware upgrade on the router, although it doesn't explain why it suddenly stopped working.
If you have another computer you can try the adapter in that would also be a good indication of whether the device itself is faulty or there is a software issue. I haven't had much experience with third-party wireless products on a Mac so thats about the best I can help.
BTW as a side note MAC filtering is extremely ineffective without encryption (unless you just have it off for testing).