Assetto Corsa is a wonderful simulation program with hundreds of cars, mods to add. Sometimes a mod or a car that is not perfect can cause the program to malfunction or even crash.

What to do?

Very simple.

Follow this path: 

Open STEAM with a click on the icon

Click on Library

Right-click on the Assetto Corsa icon

Click  Properties

click LOCAL FILES

click Verify Integrity of Game Cache...

and all Assetto Corsa files will be verified and restored.