The problem where your fans etc. starts and sound works and the screen is black, but when you shine a light at the screen you can see what it should display is caused by the backlight for the screen isn't working.
There are multiple possible causes for the backlight to stop working, it can be everything from the lights in the backlight being burnt, to corrosion from water damage at connectors or in the worst-case water damage to logic board/components.
I would start with opening the laptop up and looking for corrosion after the water damage, then clean it up using 95%+ (preferably 99%+) isopropanol and eg. a toothbrush.
Check the connectors to make sure that they're not loose.
If non of the above helped, you will probably need to replace the screen assembly. I can't promise that this will work since there could be damage to the board, but this will most likely solve the problem.
Remplacement de l'écran complet du MacBook Air 13" début 2017
Hope this helped and good luck! :D
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