windows 10 - Account has lost all of it's permissions, how do I fix that?

I made a secondary account when a guest came, and went to the C:\Users folder, right clicked on the main account, went to "Give access to" and then "Remove access". I messed with some other options as well, to make sure they can't enter.

Now, when I log on the main account, the screen is entirely black, no icons are shown, all of the folders on the taskbar are shown as white papers, and I don't have permission do anything on that account, or see anything.

I have to access the files on the main account, via some other account, by going to C:\Users\MainAccount and then typing the password of that account. And then I can access those files, but only from another account, never from that main account. What can I do to fix this?