problem with connection

I'm not sure if this is the correct category, but here goes:

When I plug in PC, the light in the back goes on, but it won't actually start up unless the ethernet light is on.
This prob has been going on for a couple of weeks, and now it won't work at all.

There's nothing wrong with the modem or ethernet cord, because it works fine in my laptop.
So it seems there's something stopping the ethernet from connecting properly to the PC.

Any idea what the prob is, and if it can be resolved?

Thank you.