What temperature would you heat frozen lasagna in the oven at?
350F is the magic temp. Just about every casserole cooks at 350, and you cook it until it starts being bubbly which will probably between 30 minutes and an hour (in this case, you should check to make sure it's hot in the center as well, since you're starting from frozen). I'd cover it early, and then uncover it near the end so the top won't get dry.