YOu haven't provided much info ,but going off of the screenshot, it looks like you have a directory called cow off of / that you're trying to mount to /root/cow and it's failing. My guess is that you updated /etc/fstab to do this. If I'm correct, then you need to boot up to a live usb drive and edit /etc/fstab and remove that line or # it so that it's not read during boot up.