I hate to have to tell you this, but backing-up that way is unreliable and not supported by Apple. See: Using Time Machine with an Airport Extreme Air Disk.
Assuming you've been backing up successfully before, and this is new behavior, there are several possibliites.
First, just re-select the drive in Time Machine Preferences.
If that doesn't help, try repairing the backups, per #A5 in Time Machine - Troubleshooting.
Then please clarify what you're asking.
You've posted in the Time Capsule section, but say you're using an Airport Extreme, as does your signature.
Those are similar, but different, pieces of Apple Hardware. They both contain wireless routers, but the Time Capsule has an internal HD that is a supported destination for Time Machine backups; the Airport Extreme has no built-in HD -- you can connect one and use it for many purposes, but not Time Machine backups.
Please provide as much detail about your setup and what's going wrong as you can, per #A9 in Time Machine - Troubleshooting.
The paperwork I have says "Time capsule is a full-featured airport extreme base station". So I thought it was the airport extreme.
That's true, but a bit misleading. "Time Capsule includes . . ." would be more clear.
When I try to do a back up using the time capsule, I have been getting a message that "Back up disk is not available"
First, just re-select the TC's internal HD drive in Time Machine Preferences.
If that doesn't help, try a "full reset" per #A4 in Time Machine - Troubleshooting.
If still no help, try repairing the backups, per #A5 in the same article.
Oh, and you might want to change your profile from Airport Extreme to Time Capsule.
Message was edited by: Pondini