Unable to read my encrypted backup files after upgrading from 12.04 LTS to 14.04.1 LTS

When upgrading from 12.04 LTS to 14.04.1 LTS by help of the update function of Ubuntu there was a system breakdown to the end of the upgrading process and I could not get access to my computer by help of my password to the already installed 14.04.1 LTS.

After that I installed 14.04 LTS from a DVD and everything worked fine until I wanted to install my files from my external and encrypted backup hard-disk (I had updated my files prior the the update under 12.04 LTS). The hard-disk was formated for Linux (FAT)

Every time I try to get access to my files now I get the following message:

Error unlocking /dev/sdb2: Error spawning command-line `cryptsetup luksOpen "/dev/sdb2" "luks-0b7f3b35-5d0f-4980-86ed-d93c8951e894" ': Failed to execute child process "cryptsetup" (No such file or directory) (g-exec-error-quark, 8)