backup disk could not be found

You cannot have an error on the Time Capsule..

You can have an error on Time Machine.. is that what you mean??

If you have an error reset the Time Capsule.. just a reboot is often enough to get the network up and running again.

If not reset it to factory and redo the setup.. it will then likely work for a few days at least.

What OS is on the computer.. ?? Mavericks would be the most likely as it has the worst networking reliability.