Wiggle the AV Cables.
Make sure the Cart connectors are clean, and you have a Game Cart plugged into the system.
Check the Jumper Pak area on the Console.
It should have
something stuck in there (Pictures would be nice).
Sometimes the Jumper or Expansion Paks go bad.
Also, try using AC Adapters and Jumper/Expansion Paks from known working systems, on the seemingly broken ones.