Cooking frozen food that was cooked
It depends on how fast you want to eat.  If it's a matter of quite a bit of time, then a safe route would be to have it in an oven-safe vessel, covered with foil, and leave it on the oven at a relatively low temperature (275 to 300)  for a long time (depends on serving size and type of frozen meal, 30 minutes to an hour, usually).  That's the method with least effort and intervention.

This will allow the food to get hot, all the way through, but not to scorch or overcook.

If you forgot to defrost and want to eat fairly quickly, the microwave, or defrosting in the microwave, and then moving to the oven is a quicker route to eating.