Urgent: Access Permission Issue

Hi, everyone, my name is Cocobeaboy and I am a new member of the Apple Support Communities, nice to meet you all.

I encountered this problem when I was just exploring access permissions of files and folders (the bottom box you see when you press "command + i" with the selected file/folder). I accidently changed the access permission (everyone) of Macintosh HD from "read-only" to "no-access". Then my computer froze and I thought restart the system would fix this, but only to find out that my computer is stuck on the starting screen.

I tried to change the permission by using the "chmod" command in the shell/root screen, and the result is this: "chmod: Unable to change file mode on Macintosh HD: Read-only file system"

What should I do? Any help is appreciated.