There's all sorts of possible reasons..... no reason to list them.
A question or two:
* What was the last thing done with the amp before turning it off the last time?
* Where has it been since then, and what has happened to it?
The very first thing to do is to look on the back and see if the circuit breaker has popped.... If so, the little lever dingus will be "open"..... push it back in.
If it resets, try the power again. Either it pops again, or it turns on.
If it pops, get it repaired, there is a problem. (could be a bad breaker, they used to be US made, but the newer chinese ones have had quality issues)
If it refuses to reset, most likely there is a problem, because the failure mode for the breaker in case of a crazy overload is to open once, and stay that way.
if the breaker is NOT popped, then anything from a wire loose, or a bad switch, onward to a more "real" problem is possible, and a reasonably decent tech is needed to determine what.
Usually, the type of problem is possible to determine from the behavior of the unit, and a few meter checks, without having to take it apart very far..... cover off, and you can see most of what you need for this type problem.
Yes I USED TO work for Ampeg...but I haven't forgotten everything.