You may have to "rescue a broken system".
Assuming the disk is good, this would be a good time to backup things before the problem gets worse. Generally people run a live or memory resident distro for this.
Things will be a little different depending upon UEFI bios being in use.
If you have UEFI bios, you will have a half gig FAT partition. You can see these with parted or even gparted from a live distro.
In any case, using a distro usb or cd, get into "rescue a broken system".
You want to use the rescue disk's kernel and "pivot root" to run your broken system. Generally you can identify the "root" partition, on simple systems it is the largest. You would mount the EFI partition as /boot/efi.
A few assumptions have been made. Re-install grub, the below assumes 64 bit and efi.
grub-install --bootloader-id ubuntu --target=x86_64-efi /dev/sdb
Apologies if you are past GRUB. There are many similar questions, even on other sites that can give you step by step instruction