Take enough time off the bike to rule training stress out of the equation. For most of us a few days off will do it but if you've really dug a hole you might need a week off and then a couple months of reduced volume. Do you train with power? That's a good way to be objective about bike-specific induced fatigue.

Once you can rule that out, if you're still fatigued, see a doctor. Focus on nutrition, relaxation and a positive attitude. Be low - be easy.

So basically + 1 to what everyone else has already said