crack under the macbook screen but the glass looks fine

Unfortunately the glass looking fine doesn't mean much on a MacBook. The front glass and the LCD panel underneath are basically one piece, and the panel can crack from pressure while the glass stays intact. Usual cause is something small left on the keyboard when the lid was closed, a pen or an earbud. If it's a dark line or a spreading patch it's the panel and the fix is a whole display assembly, not just glass. Plug in an external monitor in the meantime.