Look through the symptoms on the bulleted lists below
http://www.mayoclinic.org/diseases-conditions/depression/sym...
Do you recognize them? What are you doing instead of solving problems?
Even if it's just a dip in motivation or some low-level anxiety or something, it's worth it to work on it with a professional. Everyone has things they can work on.
Just keep it in mind. Maybe you need a vacation.
Another silent killer is poor air quality. Make sure you're getting lots of fresh air each day.
Obviously eating well & exercising helps as well.
Beyond that, simply taking a break may help.
Uninspiring work can cause sustained and unhealthy stress in the best of us. And stress is known to deplete cognitive abilities. It's probably stress.