Most people try to feel their way into better behavior. It doesn't work. Rather, behave your way into feeling better. What I mean is, do something productive. Whatever your routine is, change it. Get moving and doing. If you just have a case of the blues, then accomplishing something that you feel good about will almost always get you going again. On the other hand, if it's clinical depression (doesn't sound like it, but what do I know) then you should seek professional help. Some others have mentioned things that seem like good ideas: finish one project (I suggest a small one, and one that's kinda fun), exercise, whatever does it for you, but do something you're not doing now.

HTH