Visual Studio 2013 Unit tests don't compile - header files not found

I'm testing some c++ code for a homework, and all of the sudden, the concerned header files for code I haven't changed anything in are not being found anymore. I just added one test, and did not change anything in the concerned .cpp en .h files. I get compiling errors, but I really don't have a clue what is happening because nothing changed. I'm not including the code because I think this is irrelevant, but I one thinks I should I will. Is this some known structural problem with the Unit testing module in VS20113? I have searched the net for any similar cases, but haven't found any. If I should address this question somewhere else, please let me know.