What you have done exactly before wl500b become dead?
If your unit does not go to firmware restoration mode, then your are out of luck... So, the exact sequence for me was following: turn off power by removing power jack from the unit (you may want to wait for 10 minutes, before proceeding after that - this sometimes important), push RESTORE button (do not release it), insert power jack back to wl500. Wait for 5 seconds (at most) and release the RESTORE button.
RESTORE button should be pressed during powering unit on, this is very important.
There is a small chance, that RESTORE button does not work, that's why unit does not go to restore mode.
Also, my PCB contains RESET button on the right side of the PCB if you're looking from the LEDs. So you may want alternatively push RELEASE button, the press RESET and after that release RESTORE.
If it's does not work - does power LED blinks at least once, or it just solid?