I'm pretty familiar with the state you've described here. I usually drop into this temporary anhedonia when I haven't talked to people for a while, haven't recently accomplished anything I'm particularly proud of, and haven't gotten much exercise. (My diet is fairly consistent, so that wouldn't appear to me to be the controlling factor.)

I can escape this anhedonia by getting some exercise, or spending time around friends. Accomplishing major goals would also do it, I suppose, but that's a rather harder to control.