It happens sometimes. The problem is usually solved by full re-build. In more rare cases, it need "Clean solution".
There are more important things to it. You should periodically check up if your can build everything from scratch. You also should no exactly, what is source code and what is not. I know many developers who develop a lot of code without clear idea and late face problem of failure to build or run the project an other machine. Everyone should do this exercise on regular bases, ideally every time the structure of the project is changed.
That said… it look like you're working without any Revision Control? To me, this is completely crazy. You need to use Revision Control. I do it even on my individual projects, even with smallest one. It is easy to install and use; and the sense of security absolutely worth the effort. Which one to use? I am sure it should be Open Source. Not because of money, but because of security of your code asserts. For more information, see this discussion:
Revision control systems, which to choose from?[
^].
—SA