unknown filesystem, grub rescue

If you don't know your Ubuntu boot partition, check them one by one:

ls (hd0,msdos5)/
ls (hd0,msdos1)/

When you hit the right one, you'll get a line mentioning "lost+found" and so on.

Assuming (hd0,msdos5) is the right partition:

set root=(hd0,msdos5)
set prefix=(hd0,msdos5)/boot/grub
insmod normal
normal

Now you'll be able to boot into Ubuntu. Once you do, reinstall GRUB:

sudo update-grub
sudo grub-install /dev/sda

If you have a UEFI bios make sure you boot through uefi boot and not legacy. Grub is actually there but the bios doesn't allow it to start

The easiest way i found to fix this error was to put in an old windows 98 disk and in dos you can delete and create partitions and then format