(I'm assuming you mean it won't even respond to holding down 'c' on startup.)
The bad news is it sounds as if you killed the software in the EPROM of the phone. (This is why it can't even respond to 'c')
The good news is that you might be able to hold down the '2' & '5' keys instead of the 'c' (bypassed EPROM) to fix the software in the EPROM.
Being a relative newby, though, I'm not sure exactly how you'd do this but I'm sure others can help you here.