Firmware recovery
Revisions D1 to D4, H1 as well as I1 are pretty much unbrickable as they have a built-in firmware recovery mode. Revision D5 does not have built in recovery.
To access, simply hold down the reset button while powering on the router. The power LED should blink orange. Set your computer with a static IP in the 192.168.0.x range, and open 192.168.0.1 in a web browser. Recovery mode loads up within a few seconds, but doesn't respond to pings. If the power LED is blinking orange the above page should be accessible.
You will need to flash either a stock firmware image or Openwrt-factory image, *not* a sysupgrade image.