It's possible that you shorted it, and it sounds as if it's simply along the lines of a melted solder joint since the amplifier still works with the ground attached to the case. You can crack the amp and look around the terminal for any damage.
i already did and yeah i did find some solder that is broken along the negative that goes to the rest of component, will adding some solder to this area fix the prob? or is there something else that passes more current available?
When you say that you touched the - terminal to the metal case, you did mean you hit the case with the chassis ground, right? There really shouldn't be an issue with that, most grounds are attached to the amplifier case as well, after all, there is a reason that amplifiers are made of metal and not plastic on the outside. Depends on the amp, though. Just making sure I understood you right.
Now it's making sense, I knew touching ground didn't sound right As far as the "broken metal", do you mean a melted joint or was it a component that was broken? If the components show no damage and the solder joints are simply broken, try resoldering and see if it fixes the problem. Hopefully you didn't fry any components, transistors, caps, resistors in amps are fairly sensitive.
nothing fried as far as i can see, just the metal that runs along the circuit board.
well, if its a good amp repair it. But as you said if you touch the ground wire to the metal case it works, so in all reality you could just run a short self drilling screw into the case and use a ring terminal on the ground cable to secure it to the screw....just make sure the screw does not damage anything interally (check where you drill first) ....if the amp is anything decent i would repair it but if its a 'beeter' so to speak just do that...