Hello i tested following commands in my laptop - yeah I deleted everything in /dev/sda1 and I got it working again - so here it is:
Get a live-image and boot from it.
First lets get a clean /dev/sda1 - open Gparted; reformat /dev/sda1 with ext2 and don't forget to set the "boot"-flag (right click on the partition and select appropriate option) and confirm the changes.
Now we will prepare everything to chroot into the installed system and then we switch into it: sudo cryptsetup luksOpen /dev/sda5 sda5_crypt
sudo vgscan --mknodes
sudo vgchange -ay
sudo mount /dev/mapper/ubuntu--vg-root /mnt
sudo mount /dev/sda1 /mnt/boot
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
Okay let's delete and reinstall GRUB: apt purge grub-common (have your terminal in fullscreen-mode due to ncurses), this might ask you if it shall delete everything - select yes; now lets reinstall it with apt install grub-pc here select /dev/sda when asked.
Lastly we need to reinstall a kernel to get the needed initrd*- and vmlinuz*-images into /boot/ . We get currently installed kernels with apt list --installed linux-image-* and now we reinstall this kernel with apt install linux-image-[version-numbers]-generic --reinstall - don't forget to exchange the brackets with an actual version number.
Almost done; exit chroot with Ctrl-d and reboot!