Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
Is the model number an SA 800 or an SE A800 ? Depending on the Full model number and the age and construction of the amplifier, it may be easy to repair or it can involve the need for test equipment. Most of these amps use integrated amps, but higher powered amps may still use FETs or Transistors in their design. Is there any chance you found the Left channel's wiring was shorted, or it was run at a high volume level and the amp shut down while running?