1. Fiddle with various things in case it's just a loose connection.
2. While trying to look methodical with your fiddling, see if you can figure out how it's meant to work and search for scorched bits or booby traps.
3. Take some readings and log some part numbers and write them down carefully in an intricate-looking table.
4. Rattle off some interesting facts you've just discovered, to distract the customer while Googling the part numbers +"common fault" or +"not working" on your phone.
5. If that's no help, locate the grungiest-looking part and try changing it
6. If still dead, tell the customer that the damage is so extensive, the unit is beyond economic repair.
How many of us have done something similar under duress?