Were you doing an update from 10.5.5 to 10.5.6 or an upgrade from 10.4 to 10.5? If its a white wireless keyboard there should be a toggle switch on the underside of the keyboard. Flip it a couple times until you see a blinking green light, which means its in discoverable mode. If instead its an aluminum keyboard, one that lacks the numpad, on the right side of the keyboard, opposite where the batteries go in is the power button. Tap it once, see if it starts to respond. Also consider replacing or re-seating the batteries if you haven't done so recently.
Definitely also take a look at the Bluetooth pane in your system preferences, make sure that its active and that your Bluetooth card didn't suddenly go out of whack.