Windows Home Server not connected to server

I don't know much about computers so please let me know of you need more info.

My backup pc has WHS installed and I went to back up some data today and I got the message that WHS was not connected to the server. The machine was working fine yesterday and now I can't connect to the internet.

I've checked the modem and router and both seem to be in working order. What could have caused this problem and how might I go about fixing it?