Pull the battery and stop it now. Then make sure the battery is fully charged (if the phone keeps rebooting, you'll need to charge it in an external charger or another phone). Then follow the instructions on http://wiki.maemo.org/Updating_the_tablet_firmware to reflash the firmware. My advice would be to avoid using Nokia Software Updater to flash the phone in future - it seems to be the cause of most, if not all, update failures.