Could be an issue with the logic board (most likely) screen (if liquid spilt on bezel, where it says "Macbook Air"), or cable. The screen doesn't fail on it's own, and would only need replacing if liquid has got onto the screen PCB.
Open it and check the board for liquid damage. Clean with 95%+ isopropyl. Check continuity on the brown fuse, near where the screen plugs in (has a P or an N on it). Check screen connector and cable for burnt pins.
Beyond this, you need to get a schematic and board view for your board (should be 820-00165, but double check), have a multimeter, soldering iron and hot air rework station, among other things. If you have no experience in this, take it somewhere.
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