I would strongly suggest using a Live Ubuntu CD/Flash/Boot device to load a working image on to your computer, and then mounting that partition manually to see what could be occupying that partition. Personally I think the partition table has been corrupted, or there is damage to various sectors that make up the hard drive and the operating system routines are unable to figure things out. Perhaps an FSCK might help using alternate superblocks as a guide. What is /dev/sda5 actually comprised of (Physical, Logical, Hardware)? Is it really important to the boot process of your host?
Can you consider booting of the live image, mounting /cow (on target system) as /mnt (or something like that on the live system) and manually editing /etc/fstab to comment out the mount command for /dev/sda5 - that way your basic system (target system) will be at least be able to boot up, albeit missing /dev/sda5.
The process is difficult to grasp, but frequently done, and you will learn a lot in the process ... but will need a few tools as mentioned in : https://help.ubuntu.com/community/LiveCdRecovery, also https://help.ubuntu.com/community/FilesystemTroubleshooting, (hopefully you will not have serious problems, and can do: https://www.maketecheasier.com/check-repair-filesystem-fsck-linux/) and most relevant IMHO: Unmount drive from emergency mode