Burn a live Linux system onto a USB (can be one with just a terminal, like Arch Linux). If you don’t have another computer to plug the USB into, this can be done on an Android phone using EtchDroid.
Then, boot from that USB and mount your main filesystem. Inside of the Live system, chroot into the mounted filesystem and run sudo grub-mkconfig -o /boot/grub/grub.cfg which should fix the bootloader.
After that, you can just exit the USB system and return back to your OS which should boot now.
(If you don’t know how to mount a filesystem or chroot, I would explain but I forgot how to do it. If someone else could explain that would be neat)