updates – Unable to get backup due to GPG error

I involuntarily erased all data from my hard drive, but I have a new backup on an external drive.
Trying to restore I get:

GPGError: GPG Failed, see log below:  
===== Begin GnuPG log =====  
gpg: AES256 encrypted data  
gpg: encrypted with 1 passphrase  
gpg: decryption failed: Bad session key  
===== End GnuPG log =====  

I had to reinstall Ubuntu as well – is that the problem? How do I get my files back?