I can't urinate. It has been over 10 hours

Sooooo, Have you made an appointment with your doctor or gone to the ER yet? If you haven't when do you plan to?

List of possible reasons are large, and all can lead to serious issues. Not knowing age, health, intake of fluids, and dozens of medical issues,

Yes, time to go to doctor today or ER

One of the most common causes of absent urination is severe dehydration; you can check for other symptoms here (http://www.ehealthstar.com/dehydration/symptoms-and-signs).

Much less common but much more severe is acute kidney failure, but this does usually needs some reason: intoxication, severe muscle damage, etc.

Two other causes: a kidney stone blocking the urethra, and enlarged prostate.