Why isnt my coolant circulating in the engine?
Stuck thermostat?
If we knew the make, model, and year of the vehicle as well as what symptoms led you to look and how you came to that conclusion we’d be able to help much more effectively.
Maybe because you have not turned it on or no coolant in the engine. 
We need more information. Why do you think it is not circulating? What exactly are the symptoms. What have you done to try and fix it? How long has it been doing this? When was the last coolant change? Make model and year of car? Color? Have you checked the oil? Are the radiator cooling fan(s) turning on? What driving conditions? Etc.
Stuck thermostat? Bad water pump? Blocked coolant passages, hose, or radiator? There are many possibilities.