Sherwood audio receiver not turning on

Hello, I have a Sherwood Audio/Video Receiver RD 6504. I have had the unit and when I got it, it powered on just fine. I never used the unit and let it sit unused in a storage area. The area was in a controlled climate and shouldn't have caused any damage. When I power the unit on it goes into standby, when I press the on/standby button it goes to volume options, and then goes back into standby within a few seconds. The only controls that work when it is in this state is the volume knob. None of the other buttons or dials seem to be doing anything. When I turn it on the power supply clicks and hums but clicks and turns off shortly. Does anyone have any advice on how this could be repaired? Thanks!