what Temp and how long to reheat a cold, refrigerated, 13 x 9 casserole, in glass?
It will depend somewhat on what is in the casserole and how full the dish is. I'd bake it at 350 degrees and check on it after 20 minutes but leave time for a full hour to reheat if needed. If the top starts to get overcooked, put some aluminum foil over the top.
also, crust and outside might be rewarmed, but inside still cool-ish, sothis might be a place where a thermometer will help you judge that internal temp is right.