Yamaha HTR-5630 5.1 Channels Receiver

Worked great for the 20 times it has been used. Then, one day out of the blue, I turned it on for a while, then turned it off, then a while later decided to turn it back on and it goes off by itself after 3 seconds. I tried to plug it into another outlet but still the same thing. We have had no power surges, wasn't turned up loud, the surge protector is a Monster brand one (paid over $100.00 for that surge protector) and every thing else connected to it works fine. I've tried contacting Yamaha, but no answer by phone or email. I own several of their products but will be getting rid of them to purchase another brand....I've had great luck with Pioneer, Harman Kardon, and Sony. This receiver has hardly been used and was the one in our bedroom. It should be still under warranty. (I sent card in when purchased but seems to be missing) and I can't find the receipt.