To fix the issue, you need to regenerate the initramfs for the new kernel version. Run the following command in the terminal:
sudo update-initramfs -u-k <version>
Replace <version> with the actual kernel version string for the kernel that you were unable to boot into. For example, it might look something like 4.15.0-36-generic.
You can find the kernel version by running uname -r if needed.
Update GRUB
Once the initramfs has been successfully generated, update the GRUB bootloader by running:
sudo update-grub
This command ensures that GRUB recognizes the updated kernel and its corresponding initramfs.