LUKS on Ubuntu 16.04 : unknown filesystem type 'crypto_LUKS'

To be able to access your files you have to decrypt them first

cryptsetup open /dev/sda1 backup

where sda1 is the encrypted partition and backup is the name of your partition (that is a very basic command, you can fix it according to your needs). Then type your password, and proceed to mount that unencrypted using

mount /dev/mapper/backup /mnt

Done! now visit the mounted partition

cd /mnt/backup