This WRT54G worked fine for a little over a year and then one day it wasn't lit. I went through all the normal checks from the wall outlet to the output of the adapter, no problem. Opened it up and tested the fuse, no problem. Traced the supply voltage as far as I could without a schematic and had to stop there.
Did you try doing a Hard Reset of the Router by unplugging the power cord and holding the reset button in for 30 seconds then plug it back in to see if it does anything?
If this should work, then you'll need to log back in and redo your configuration and be sure to set your Administrator Password (a strong one, not weak). Plus don't forget to set the password for the User access if there should be such an account as you don't want to leave this open either.
If the Router fails to respond to a hard reset, then it might have become bricked.
Possible causes could have been a power spike, a hacking attempt (doubtful), or one of the newer worms which attack vulnerable routers by flashing the firmware with a Botted firmware making your router part of a BotNet. The last could only have happened if you either left the router password at factory defaults or if you used a weak password (dictionary attack). A failed attempt at flashing the firmware with a Botted firmware would explain your Router being Bricked. (if it is bricked)
The reset button does nothing while the router is unplugged, of course.
Holding in the reset button for at least 20 seconds, after the router is already booted up, typically clears saved settings from NVRAM; holding in the reset button while applying power and continuing to hold in the reset button for at least 25 seconds usually completely clears NVRAM, though it can have different affects on different routers and firmware... ergo you should not do the latter procedure on a router with its stock firmware without being advised to do so by a technician.