The off and unplug trick is a good one to try--if it's the firewire bus and not the hard drive that's the issue, that may help (before letting it sit you might also press the power button on the computer after unplugging it; the light should go on briefly while it bleeds off remaining stored energy).
If, however, it's the drive acting up (do you have any other firewire peripherals to test the port?), then this isn't going to help. Open Disk Utility and see if it sees the drive when it's plugged in; if it does, then the case is functioning anyway. You may be able to repair it from there (though I expect you'd have gotten errors already if it was that simple), or at least reformat if its hopeless.
Generally speaking the most damage that should've been done is to the directory on the external drive; it shouldn't have broken anything (although the firewire ports on older Macs are apparently more prone to power issues when "hot plugging"). I can't imagine it having done any physical damage to the external, anyway.
