My Steam gives an "Failure to start game Unknown Error" on games that are already installed and I've played it earlier in the day

I've had similar issues with games, especially Skyrim, and anything I'd played that day would no longer work. I ended up having to restart my PC each time it would appear otherwise I couldn't start my games. Typically I would get it if Skyrim crashed or I cancelled the loading sequence before it was finished.

This blurb is taken from the Steam forums (granted is for a separate game):

Solution 1. Run the game as administrator
Find the file SalvationProphecy.exe

On my computer (win7, 32-bit), it is located here:
C:\Program Files\Steam\steamapps\common\SalvationProphecy\bin\release\SalvationProphecy.exe
Right click on it
Select "Run as Administrator"
If that doesn't work, go on to the next test.

Solution 2. Run steam as administrator
Exit steam completely: (top left menu) -> Steam -> Exit
Wait a few seconds for it to finish shutting down
Right click the steam icon
Select "Run as Administrator"
Try running the game normally
If that doesn't work, go on to the next test.

Solution 3. Verify the integrety of game cache
Run Steam
Right click on Salvation Prophecy
Click "Properties"
Click the "LOCAL FILES" tab
Click "VERIFY INTEGRITY OF GAME CACHE..."
Wait until it is done
Try running the game normally.
If that doesn't work, go on to the next test.

Solution 4. Check your antivirus settings
I can't give precise instructions here, because it depends on your antivirus program. But sometimes they can block a program from launching. There has been a problem reported with Avast.

Solution 5. Restart the computer
The old classic, always worth a shot.

Source: https://steamcommunity.com/app/248450/discussions/0/792923683573395036/

• Please try out the steps shown on the Steam troubleshooting page.
• If this doesn't help, please look up if the task really is killed by entering the task manager.
• You can also try to start the game in the offline mode as shown here. (Please be aware that some applications needs to sync with Steam by time.)
• Restarting your PC always is an option.
• If Steam is set as auto start, please remove it, and start it manually.
• Please check whether games are blocked by a firewall or similar programmes.
• At last, you can reinstall Steam, too.

Check if the problem occurs on other machines too. Doing so, you can ensure that the problem is specific to your current machine, or to the software itself. Either way, you can always create a new ticket to get some support by Valve employees.