Steam game (DOS2) shows as uninstalled

If you shut down your computer too fast, steam sometimes corrupts certain files that help steam recognise that the game is installed. Those files are called app manifests. If you want to prevent this from happening, don't force turn off your computer (holding power button) and let steam finish closing. If the problem still continues after this, go to the games store page and remember the ID number at the top then close steam. Go to your steamapps directory and find the app_manifest with that number next to it. After you've done that find someone who already owns the game to copy their manifest, or find it online (the most recent one) and replace the text in the manifest file with theirs/ the one you found, and then save. Then open steam and it should be installed. Should take less than 5 minutes the second time.