First, I would check the obvious stuff. Wires, jacks, pots, and switches for bad solder connections, loose wires, burnt or hot spots on the pc board, etc.... You might also try connecting everything with the unit opened and turned on. Take a plastic stick or Popsicle stick and try pressing on the PC board in various locations. Sometimes you can pinpoint a bad solder connection this way. If it suddenly works, reflow the solder on all the connections in this spot.
If these look OK, get a volt meter and check the voltage at the power supply.
Reply back here with your results and good luck!!