You're most likely booted, otherwise you might need a live USB. Hopefully, the system isn't in read-only mode. What I'd recommend doing is:
cp /etc/fstab /etc/fstab.backup
To make a copy once. Then, nano /etc/fstab to run nano, a basic CLI editor. You can use the arrow keys to navigate and type freely in it. The hints like ^O shown on the bottom mean ctrl+o.
You'd use the arrow keys to go down to the line that probably says /dev/md0 /mnt/raid morecrap, put a # in front of it, press ctrl+w then enter to save. If that worked, ctrl+x to exit and try a reboot again.
Obviously can't promise this is "the" error preventing the system from booting, but it's generally a good idea to disable broken stuff like this to get the system working again, then fix it from there. Hopefully, this does the trick. Your RAID setup will not be activated on reboot after you do this but it's not going to permanently delete data or anything.