Hmm.. try going to the "library" section of steam, right click on the link to "IL2: Cliffs of Dover" and select "Properties". Then go to the "Local file" tab in the window it brings up (along the top) and click on "Verify integrity of game Cache".
Not sure if it would work, but it's worth a try!
Sorry if that is a little patronising, explaining it simply is the only way I can tell my sister how to fix things xD And
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V.A.S. Echo Flight : Flight Lead
PSN's: P-51D5 and Reaper1526 
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