My common solution to boot issues that lock me out is,

1. Get a USB Live distro (always good to have a backup lying around)

2. Boot into live distro

3. Chroot into the broken distro (Arch wiki has an article on this)

4. Fix any bugs that I can find and/or reinstall the bootloader

5. Test, repeat until the problem has been found and fixed

Hope it helps any newcomers!