as chris said, also for whatever reason (powerdown after a long time), its also possible its firmware has become slightly currupted, so again perhaps a re-install/update of its firmware might put it right or perhaps even better if you have the right HW versions, install a 3rd party firmware
OpenWrt or instance
http://en.wikipedia.org/wiki/WRT54G
http://wiki.openwrt.org/TableOfHardware?action=show