Whenever I have art block I think of something my teacher told me, where he said that whenever you feel like your art took a nose-dive, and everything is being drawn by a kindergartner--that means you are about to improve in a big, big way. What happens is you become good enough at art, that you recognize what you want to fix in your own pieces--that's the slump. Everything looks bad, not because you're suddenly bad, but because you became aware.
Getting out of it is just patience and taking a little time for self improvement. You will draw a thing eventually and be like "hell...I drew that?" It just takes time. Sometimes it takes walking away from it so you stop being so critical of your own art (and I do think that learning not to be critical of our art is a learned skill), and sometimes it just takes just drawing and being OK with it not being at 100% because you are where you're at, and that's OK. But, the art slump will end because you will improve to the point that you will be satisfied. That's the way of things. You kind of just have to trust the method.
As others have said, you don't need to be always producing stuff. I have a tendency to just stop producing when things get rough. Sometimes that time away helps to reset bad habits I may have had in my own art process, so I can start over again with good habits.