Linksys WRT54GC v2.0 - all connection lights flashing

Your WRT router's power supply is failing. They used low quality capacitors inside it and they became swollen.

The output voltage changed making your router behave abnormally. Try using another power supply for it and it should be fine – if you still have it.

Sounds like you may have a bricked router. You could try again to hold the reset for longer than 30 seconds, and make sure your computer that you have attached via Ethernet to port 1 of the linksys is set to automatically obtain IP via DHCP.

If the multiple 30 - 60 second resets are not working to bring it back, there is an option of flashing but without some sort of connection to it I doubt that would work either.