aside from the obvious re-checking of all connections when you reassembled the machine, have you tried disconnecting the PRAM battery for a few minutes and then rebooting?
if the PRAM battery is dead, the G3 Pismo may not start up as long as that battery is plugged in - the machine will appear completely dead. often unplugging the PRAM battery will let it reboot.
this and further troubleshooting ideas can be found [http://www.ifixit.com/Guide/Wiki/PowerBook_G3_Pismo_Troubleshooting|here]