Is the TV upstairs wired or wireless? My "read" is wireless but want to be sure.
Verify that that TV's network configuration still matches the router's configuration for your network.
Does not take much to degrade a wireless signal - either moving the router or the device. Does the TV start working if the router is placed back in its' original location.
Make sure any ethernet cables along the network path are correctly connected. Try using another known working cable as a replacement. Could be some plug/unplug caused a connection problem within one of the cable plugs.
Are any other devices affected?