I got into the same situation you did. There might be a simpler way, but this is what I did to fix it. You'll need an Ubuntu USB so that you can chroot into the encrypted partition.
You need to install cryptsetup-initramfs and then run update-initramfs by doing the following.
Chroot into the encrypted partition
Boot into Ubuntu from a USB and click the "Try Ubuntu" button. Open up a terminal.
Find your encrypted partition.
sudo fdisk -l
Decrypt the partition. Let's say my encrypted partition is /dev/sda3
sudo cryptsetup luksOpen /dev/sda3 sda3_crypt
It should prompt you to put in the password to your encrypted partition.Now we need to mount and chroot into the partition. You'll need to know the location of root, as well as the /boot and EFI partitions. You can use sudo blkid to look for them. It'll look something like this where sda2 is the boot partition and sda1 is the EFI partition.
sudo mount /dev/mapper/vgubuntu--studio-root /mnt
sudo mount /dev/sda2 /mnt/boot
sudo mount /dev/sda1 /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
sudo grub-install
update-grub
Now you're in a chroot terminal, which is kind of like being in a terminal inside your encrypted partition. You should have access to your files so, before you do anything else, you can try copying them to a safe location just in case the next steps don't work.
Install cryptsetup-initramfs
sudo apt-get install cryptsetup-initramfs
Check crypttab file and update initramfs
Check the /etc/crypttab file with an editor to make sure it's set up properly. Mine was unchanged, but you might want to take a look.
Update initramfs using this command.
update-initramfs -k all -u
The -k all option specifies to update it for all kernel versions. You can specify the one(s) you want if you don't want to update them all. The -u option is to update existing initramfs rather than create new ones or remove old ones.
Exit the chroot, unmount, and restart
To get out of the chroot environment and unmount your partition.
exit
cd
for i in /mnt/dev/pts /mnt/dev /mnt/proc /mnt/sys /mnt/run /mnt/boot/efi /mnt/boot /mnt; do sudo umount $i; done
Restart the system and remove the USB. Hopefully this fixed things for you as it did for me. The big problem was that cryptsetup-initramfs was missing. It might have been removed when you uninstalled the initramfs package.
