How to Reheat Beef Tenderloin?
Bring it back to the old school, which is place a covered plate in a 125-200 degree oven until just warmed through.  It was the option of choice pre microwave, and still the best way for roasts and pastas IMO. It should come out steaming after about 20 minutes, depending on temp.  You lose less moisture the lower you go, but it takes longer so you get to choose the median point between those that's good for you.