Like mentioned above, you can just reset the processor, OR if you'd like to have the CTL-A-A functionality back, check this out:
http://mylinuxisp.com/~jdbaker/oldsite/images/A5h_A2k_CDTV_kbd.gifI'm working on rigging one of my 500's with this little circuit right now.
Of course, you DO need to know which end of the soldering iron to hold onto!