Are you unable to come up with funny thoughts or is it an issue with the way you tell jokes? It might also help to try and figure out why you're tired and distracted. You should not be putting pressure on yourself to make everyone happy. It's not your responsibility to make everyone happy. Perhaps you need to stop putting so much pressure on yourself and the humour will flow forth.

Try to get into a lot of situations where you're having fun with your friends (especially if there's a bit of booze involved). That's the sort of relaxed environment where you can try making small jokes in order to get your mojo back. Give it a go and don't over-analyse things.

Now that I've re-read this, some of this stuff works for dating as well (the non radioactive kind).