Slumps are a part of every sport. When I feel like it's happening, I move my focus back to the fundamentals.
You also have to deal with the mental aspects. If you are thinking about the wrong things (negative thoughts) when you are playing, that can cause problems as well. Play each shot, and try not to worry about the outcome. Do what you know.
I don't know your skill level, but some of the better players I know can have a bad night. Frequently, I see them trying to do too much. If you recognize that your game isn't in top form, quit trying to power draw the cue ball 3 rails, (or whatever) and just focus on making shots and trying to use the natural roll to get position. Remember, if you don't make the shot, the greatest shape in the world still means you are heading to your chair.
Steve