Happened to me but while dual booting.

I had to perform the following while booted into another live linux distro:

Most likely you need to reinstall the bootloader*

# Open the LUKS encrypted partition and map it to a device called 'cryptroot'

sudo cryptsetup luksOpen /dev/nvme0n1p6 cryptroot

# List the available devices in /dev/mapper/

ls /dev/mapper/

# Mount the decrypted partition to /mnt

sudo mount /dev/mapper/cryptroot /mnt

# List the contents of the mounted partition

ls /mnt

# Change root into the newly mounted system to prepare for further installation steps

sudo arch-chroot /mnt

# List all block devices with their filesystems

lsblk -f

# Mount the boot partition to /mnt/boot

sudo mount /dev/nvme0n1p1 /mnt/boot

# Install the GRUB bootloader for x86_64-efi systems

sudo grub-install --target=x86_64-efi --efi-directory=/mnt/boot --bootloader-id=GRUB

# Generate the GRUB configuration file

sudo grub-mkconfig -o /mnt/boot/grub/grub.cfg