There may be cases when newly activated plugin or theme breaks the website and you can't use admin to remove the extension that breaks your website. You can fix this by entering the safe mode. Just add /?safemode=1 at the end of the URL and you will open the page in safe mode. Expect styles to be broken as theme is disabled in safe mode.
Safe mode can be used only by the administrator. Make sure you are logged in to the admin before trying to enter safe mode.