Hi

Hopefully this is an easy question.

When my PC has its Cat5 cable plugged in it wont start at all no lights nothing its dead.

Unplug Cat5 cable, switch on everything fine, boots normally.

I can put the plug back in at any time and everything works fine.

Has something messed up with wake on lan maybe? (red herring alert here)

I can't remember anything I've done to cause this.

Thanks in anticipation.

Can't tell you much, since you've provided no details that would let anyone here help you. Things like version/distro of Linux, what kind of hardware, what kind of NIC (integrated into motherboard? PCI? USB?), home/office environment, etc. We can't guess; read the "Question Guidelines" link in my posting signature.

First thing to try would be to remove the NIC (if possible), or disable it in BIOS, and see if you'll boot then. Disable WOL settings and see. Do SOME troubleshooting to see if you can narrow things down. If you've not changed ANYTHING on the systems OS, that leaves hardware going flaky.

I assume an ethernet adapter built into the motherboard? Anything special about your LAN configuration?

Have you tried swapping cables or tried another port on the other end i.e. switch/router etc?

You can try disabling WOL in the BIOS if possible to see if that makes a difference.

Using any power over ethernet devices?

Thanks both, some good ideas there.

Also check if PXE is enabled, since if PXE is enabled and before hard drives in the boot order... it maybe trying to DHCP, more likely it is a Wake on Lan setting since PXE should still display something on screen.

A tough one.

I get the feeling I have seen this before in my work place.

I'd plug in a hub or switch or another computer to this nic and see if symptoms change.
Some testing of some sort to find more symptoms.

Guess it could be some wiring or nic card issue that is causing power problems.

Did you try unplugging the other end of the cable from whatever it is plugged into? What is it plugged into and has the configuration of that changed?