So look around back, and see if any tubes are glowing when the thing should be "on". Orange glow in middle of the "stuff" inside the tube.
No glow..... bad fuse, loose power connection, etc, etc. bad fuses can be replaced, ONCE.... if they "blow" again, take to tech. Otherwise take to tech if fuse is not bad, or if replacing does not fix it.
Tubes glow..... and amp is not switched to standby....... loose power connection, etc, etc. Take to tech.
Basically, if you are asking, the answer is "take to tech". Otherwise you would instead be asking something more like "OK I have a loose wire, but no schematic, where does it go?"