ZyXEL Communications Unified Security Gateway ZyWALL 1000 User Manual page 653

If you want to reboot the device without changing the current configuration, see
page 649.
1 Make sure the SYS LED is on and not blinking.
2 Press the RESET button and hold it until the SYS LED begins to blink. (This usually
takes about five seconds.)
3 Release the RESET button, and wait for the ZyWALL to restart.
You should be able to access the ZyWALL using the default settings.
ZyWALL USG 1000 User's Guide
Chapter 50 Troubleshooting
Chapter 49 on
653