I've had that happen with my WRT54 as well. Sometimes that will happen if your modem's internet connection is on DHCP. If it loses it's lease or the modem resets for whatever reason (power failure or low signal, or whim of your ISP), it will come back on with a different IP address and your router may not know the new IP. The typical recommended solution (by my ISP anyway) is to power down the computer, power down (unplug) the modem, and power down the router. Then reboot the computer, power up the modem and when the modem stops blinking, power up the router. Should be good to go with all three devices on the same page. You may also need to log onto the router and "Renew DHCP lease". Default setting for the router is 4 days IIRC.
edit:
Oops, I see you're on DSL, so you likely don't have DHCP. I'd try it anyway...