Linksys wrt54g ethernet problem

I've had this modem router for about a year and had no problems until a couple of days ago. Connections via the ethernet ports have suddenly stopped working. Sometimes when the router is first turned on it works OK, but then the connections fail: the PC says that a network cable is unplugged (it isn't) and the router can't be pinged. Wireless connections are unaffected.

It may be a co-incidence but there was a lightening storm overhead on the evening the problem started.

Any suggestions for investigating what's gone wrong? I'd try a factory reset, but if that didn't work I'd have lost the wireless connection too.