How long and at what temperature should I heat a pan of frozen lasagna?

Being a reheated food, you should warm it to an internal temperature of 165 degrees. Use an instant-read thermometer to take its temperature at the very center, where it will reach temperature last. If you don't have a thermometer, it's a fairly inexpensive investment and a great tool to have on hand. It will safe you lots of guess work.

I would recommend an additional 30-40 minutes in the oven, with the same temperature (probably 350). Leave the foil on until the last 30 minutes.