The only problem that I have come across which didnt let PS doesnt let u use filters is the DPI settings when u make a new document - Its usually 72DPI but anything higher maybe thats why If not that then.. maybe some other members can help - DarkStar
Have you made any changes recently? You have to be a little more specifik, also you should tell us which ps verision you have.
i'm almost 100% its because you chose something other than 8bit beside the color mode drop down menu.
O yesh that too Forgot about that - If its 16bit it doesnt work either so 8Bit is normal too ...Forgetful - Cant remember all problems can I - DarkStar