You really want to try to get help here first before wiping out important data. When everything else fails, you can always boot with any live DVD (don't even need to be Manjaro) you can then chroot into your system and copy/save data or most often fix your system. Especially if it is not just hobby computer!
I suggest you check out few step by step tutorials how to help yourself when you get into trouble, like this one:
This one is more detailed, has few extra steps (more mounting)
https://wiki.manjaro.org/index.php/Restore_the_GRUB_Bootloader
Don't forget you need to back up your data. Here is one guide:
Get somewhere spare/testing computer and train upper steps. So when something bad happens, you are ready.