In Windows Explorer, try browsing to the '..\SteamApps\common\Divinity Original Sin 2' folder, and check if the install files are still there.
If it is just a problem with Steam (if the appmanifest_435150.acf file in the SteamApps folder is deleted or corrupt, that will cause Steam to list D:OS 2 as not installed), re-installing should get Steam to check the folder first, usually recognise the files are, and fix it without having to redownload everything (though sometimes it still does).