I have a wrt54gc. It's been working well for some

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Have you tried hitting the reset button with a paperclip? That will reset the router to it's default factory settings after that follow the directions for first time setup

A web search netted this discussion with a couple of possible fixes:

http://community.linksys.com/t5/Wireless-Routers/No-internet-connection-when-connected-to-WRT54GC-router/td-p/27153

Has your modem been hard reset recently? if so, it may require that you bridge the modem out to work with the router- ESP. if you use Verizon DSL. If so, contact your ISP, and ask for assistance in bridging out the modem.

I’m happy to help further over the phone at https://www.6ya.com/expert/susan_42b401e0b1603d8a

double check whether you have the ethernet wires back to front

check by connecting directly your computer to modem, if still same, then contact ISP, might be a problem at thier end.

For that you need to reset your router to factory defaults.
Then reconfigure fix this problem.
First RESET your router to defaults.
for that press RESET button for 10 seconds RESET button is in back side of router.
Then connect your router to computer with ethernet cable.
Then you can access it with following link.
http://192.168.1.1
or
http://192.168.0.1
type user as "admin" and password as "admin" or blank or "password"
Then you can see configuration page of router.
Set WAN settings as your internet service provider settings.
Enable DHCP .
Click on wireless and enable wi fi.
If you want secure router then enable WEP security then add new code it is your password key for connect wi fi.
Also change channel type in wireless option 4,6 and 11 channel are best.
Try 1 by 1 then save settings and restart router.

Let me know if you need further assistance.
Thanks.

Sounds like a PSU failure possibly.

If you can still get wired internet then its not dead. Its even possible the power LED is pooched if you can still use the wired connections.

possible the formatter board has blown.

If your router's internet light is red it means to say that there's no connection. You really have to call Tesco.