Can't add new post

Try using a different browser. If it works, go back to the original browser, clear the cache, close it and then reopen it.

Try disabling all WordPress plugins and trying again.

If you click on the “Text” tab right above the editor, can you type there?

As you said,you were doing something in Settings. Probably you have changed permissions for admin user.Are you able edit a Post?

This is a very strange problem.Please check if you are using latest version of wordpress and update the plugins if you are using old versions.