Usually the backup is either unreachable or corrupted in some way.
So I think it is better to first of all reboot the whole network..
With everything off.. turn on the modem.. wait 2min.. turn on the TC.. wait 2min.. turn on the computer .. wait until everything is running.
Test and see if it works.. if not..
Click the TM icon on the top menu bar. Then hold down the option key and select verify backup.
Do this with ethernet connected rather than wireless if at all possible.