Games that were previously installed and playable are appearing as not installed in my Steam library. What can be done to cause Steam to recognize the installed game files again?
The information in this article also applies to cases in which a game's download is being reset.
Games may appear uninstalled when Steam no longer recognizes the installation files. As long as the game folders are still in the proper location, attempting to launch a game will prompt a download. This will allow Steam to recognize the installation files during the download process.
You can ensure that the game folders are in the proper location by navigating to your default Steam installation:
C:\Program Files (x86)\Steam\Steamapps\Common
If you had previously installed the games to an alternate location outside of your main Steam folder please try the following:
Once you have added the new Library folder your installed games should appear in your Steam Library.
If Steam does not recognize your games after completing the instructions that are listed above, it is recommended that you completely reinstall the games in question.
External hard drives are not recommended for use with Steam or Steam's games. Aside from many potential performance issues, external hard drives may connect or disconnect from the computer at inopportune times as part of their normal operations. If you encounter this issue with an external drive, install Steam and your games to an internal drive instead.
To ensure that you do not encounter this issue, always allow Steam to close properly when shutting down your computer. Allowing Windows to force Steam to close when it is writing to a hard drive may cause Steam library and installation information to become corrupted.
Additionally, third party applications can prevent Steam from writing essential information. Ensure that none of your installed applications are interfering with Steam: