Which Yamaha amp is it? I had a problem a while ago with my RX-V667 which was resolved by doing a factory reset and then installing the latest firmware. There's a series of 'button press' steps to do this; I can't remember them off-hand but I have a note of them (somewhere).
In my case, the amp appeared to switch on but no input could be selected, so it was in effect in a standby mode. I got great help from the Yamaha team - there's a link on their website. But if it's only 2 months old and you bought it from a dealer, the first recourse should be to contact them.