[azernik]

Take a break! Sleep, go out to a park and lie down in the sun, whatever makes you feel like you're not struggling to motivate yourself. Don't do leisure activities like computer games if they make you feel bored and unfulfilled - that'll just make you lethargic and depressed, in my experience.

If this is a regular problem for you, the time to address it is when you're feeling "up" - get exercise, get enough sleep (even if that involves midday naps, and I'm sure many other things that people can suggest).