One thing that used to give me fits, is when my PC would suffer a power loss, it would go off an get a new IP address thanks to DHCP. The problem is that you had to configure the SB to point to the IP address where the media server was running.
An easy way to check: Open a command prompt, and type: "ipconfig" and hit enter. You should see something like:
"IPv4 Address . . . . . . . . .: 192.168.1.XXX"
Check to make sure the "XXX" matches what you have your SB configured to.
If they are different, then the quick fix is to update the IP Address in the SB to match that of your computer. Long term fix is to go into your router, and create a static route for your PC... essentially creating a fixed IP address associated with the MAC address of your PC.
Hope this helps!
-DD
EDIT: I realized after the fact that Rich already answered this above.
Oh well... call it "theory reinforcement"?