Not sure what has happened. You can use Steam to check the integrity of the COD files on your PC. If it finds any issues it can replace files to get things running again.
Open Steam (online), go to the Library section, right click on the il-2 Cliff of Dover entry and you'll get a menu (first pic).
Click on 'Properties' at the bottom.
Then click on 'Verify Integrity of Game Cache' (Second pic)
Let Steam do its thing. If it finds any errors it will download some files.
If that doesn't find anything, then I think (like myself and many more people here
) that you will have to hope the devs come up with the goods in the patch. Supposed to nearly double fps.
Hope you get it working.