Corrupted files can also cause a multitude of other odd game bugs.
To fix corrupted game files, you can have Steam validate the game’s cache on your system.
In the properties window, click the Local Files tab and click the Verify Integrity of Game Cache button.
If Steam itself is failing to launch or you still experience odd issues with games not working properly, you might want to try deleting your Client file.
To ensure that you do not encounter this issue, always allow Steam to close properly when shutting down your computer.
You’ll find this file in your Steam directory – C:\Program Files (x86)\Steam by default.
However, this may not always solve the problem – if the problem has already started occurring, the gameplay may be glitchy until you relaunch the game.
One trick you can use is setting Steam.exe’s CPU affinity instead – when you launch the game through Steam, it will inherit Steam’s CPU affinity and launch with the correct CPU affinity.
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.