Well sudo is on and Su is off by defualt in Ubuntu? I don't enjoy that, Try to use command
sudo -i
if you can't log in there than you may need to reformat find a way around. I know chntpw is a great daemon to change passwords if you forget. However I only used it on windows partitions.
My advice is to reinstall, would be fastest.
Chrooting can be a good method, not sure what links to make to change password but it's quite simple. You can chroot into encrypted partitions too, just can be a bit of a pain and you need to know the pass phrase.
You are using MBR? a gtp, efi boot? Just reinstall if you want help with efi set up I can get you started and you can triple or more boot.
Grub sits on the Master boot record or in a boot partition. Typically it can even sit in the root directory , with an efiboot loader pointing the uefi-bios towards grub.
Pick a distro and reinstall, trust me it's pretty easy. Just back up your stuff, if you can't from user space than run a live USB..
trouble is you need root to run dd command (I'm sure you can do a quick google how to set up a iso into a live usb on linux) just input and output , maybe blocksize.
So if you have a windows pc you can also run rufus, If you do have a live usb boot into it, type sudo -i or su and enter the default password, usually it's just root or my favorite toor. Than you can just back up to a usb or whatever extra partitions you have.
I like to separate my home and root for this exact reason, also encryption can by a bit troublesome but it does maintain integrity.