Your amp is perfectly fine, just a matter of fixing/replacing the input jack.

The jack looks like this on the inside :



If someone bent the cable inside there and it doesn't work, chances are that they bent that long piece of metal that looks like a hook (see above.) If thats the case, you can either bend the hook piece back to its regular position, or just replace the jack. Its best just to replace the jack since its hard to get them back in the right position. To get to the jack, you need to uncrew the nut that is on the front of the jack (where you plug into.) If after you do that and you don't have enough room/wireage to work with, then your going to need to take out some screws to get to the inside of the amp. But doing that can be dangerous even when the amp is unplugged since there can still be a build up of electricity.