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?

Begin the installation process to cause Steam to recognize existing files

Games may appear uninstalled when Steam no longer recognizes the installation files. As long as the affected game's folder is still in the proper location, attempting to launch the 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

Add previously existing alternate installation libraries/folders to Steam

If you had previously installed the games to an alternate location outside of your main Steam folder here's how you can add them to your Steam Library:

1. Launch Steam and go to Steam > Settings > Downloads and click the Steam Library Folders button. This will open the Storage Manager.
2. Here you can see manage your game installations and locations. Near the top of the Storage Manager, click the "+" button to add another folder of your installed games.

Once you have added the new Library folder your installed games should appear in your Steam Library.

Reinstalling games

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.

Preventing this issue

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:

Programs which may interfere with Steam