Pressing the reset button for several seconds (keep the button pushed, until the light stop flashing) at the boot time (push the button first, then power on the machine, may need 30-60 secs) lead to openwrt safe mode.
after successful reset, 703n ip will be 192.168.1.1/24, use telnet to connect (no password). type in "firstboot" and enter, "reboot -f" and enter, then you got the original settings (192.168.0.100/24 and empty password).
the reset button can be accessed using a ball pen or something thin and solid to push in the small hole, the right side, near the ethernet port.
More details:
http://wiki.openwrt.org/doc/howto/generic.failsafe