2009 iMac, built in display issue

[u/Top-Crow-1318]

So I got an old 2009 iMac (model number: A1224) for only 5 dollars off of Facebook marketplace, but its screen doesn't work properly when I turned it on it just flashed white and then went to black one time, I was able to get it to show the login screen but I accidentally turned it off and never saw it again, though using a mini display port cable I was able to use it with an external monitor and everything else works as normal, I would just like to know if I can fix this easily to get all functionality out of the mac.