Increase or Change?

I've been on 20mg Prozac for about 5 years now except a 6mo period in the begining where I tried to go off the meds because I thought I was "cured" (didn't work :-/ ). Anyways recently the depression has gotten much worse or the Prozac has seemed to stop working. Can that happen after taking the medication for so long? I was wondering what would work best, talking with my doctor to increase the dose (since I think 20mg is a relatively low dose) or switching to a different medication? Any experiences would be appreciated.

-Ren