Or throw the whole casserole dish in the microwave - a minute for each cup of casserole. Check the size of the container= make sure it's a microwave safe dish (glass, Corning-wear , Anchor, Pyrex). Make sure it's thawed, as in above answers and when you're done, stick a clean finger in to check the temp.
You will need to thaw it overnight and set it out at room temperature for an hour. Place it in a metal baking pan and put it in an oven that has been initially preheated at 350 degrees Fahrenheit. Serve it immediately after 15 minutes or more in the oven.