Can't find stdAfx.h

The .NET project I have has suddenly started giving me the message: fatal error C1093: API call 'ImportFile' failed '0x80070003' : ErrorMessage: The system cannot find the path specified.

When I look at the code it is pointing to, I see:

  #include "stdAfx.h"
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That file hasn't gone any where. It is still in my project. Why would .Net suddenly not be able to find it? Did I screw up the search paths somehow? If so how do I reset them?

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For anyone looking for a solution to this kind of problem: select your project, go to Build menu and select Clean Solution. Then build it again.