looking for bacup disk

I have a Time Capsule, an iMac and a Mac Book Pro. Recently I noticed that the automatic backups to the Time capsule started to fail and that the backup drive could not be found. In finder, I can see the drive and can access files but when I try to run the backup from the Time Machine utility, it just waits looking for backup disk. Also can't use Time Machine so don't have any access to backup.