You will have a hard time getting help with a title like that. People won't understand what it's about.
It can be many things. I have a Marantz at my work that often goes into protection mode. In order to start the amp again, I have to unplug it, leave it a couple of moments and then plug it back in and power on. If for whatever reason your amp went into protective mode it won't power back on right away.
If you have a manual, search for protective mode and follow instructions. If it is protective mode, you should try to find out why it happened. There's no sense in just powering it back on and wait for another down time. (I have surges in the grid and a lousy set up job, but it's out of my hands and the owner doesn't care so I just leave it, but I would never advise you to do the same)
