DIAGNOSIS Main control board failure. This is the most common cause of a completely dead unit with a blank display.
FIX Unplug the microwave for five minutes. Plug it directly into a known-good wall outlet, not a power strip. If it's still blank, the control board is likely dead and needs replacement. Part number WR55X10942 for the main control board.
IF THAT DIDN'T WORK Check the thermal cut-out on the magnetron. It's a small, usually white, disc-shaped component near the magnetron. If it's tripped open, it cuts all power. You'll need a multimeter to test for continuity.
HEADS UP Replacing the main control board yourself requires disassembling the outer case. You must disconnect power and discharge the high-voltage capacitor first. This is not a beginner repair.