this is a very easy fix first and for most be very careful when messing around inside any amp they contain lethal voltage that can kill you.
buy a replacement cord( 3 wire type) take old cord out, place new cord in, on the inside of the amp use a screw thats closest to the cord as possible and use that as your ground wire, then hook the other 2 wires up accordingly and your done, be sure to take the polarity switch completely out of the power loop to insure safe grounding