Unfortunately, this message usually indicates that the backup file has been corrupted. Let's see if that might be the case.
Please hold down the option key on your Mac while you click on the Time Machine "clock" icon at the top of the Mac's screen
Select Verify Backups
Run the diagnostic check and let the process run until it completes or until you see an error message appear on the screen. If an error message appears, please post back with the exact text of the message.
The check might take anywhere from 10-60 minutes or more depending on how much data is stored in the backup file. You can watch the progress of the check by clicking on the Time Machine "clock" icon and the clicking on Open Time Machine Preferences.