In addition to the solution by Dave: if what is described in the link he provided is the reason of your problem (which is very likely), you have some other problem: you need to get a habit to build your project from scratch on regular basic, as a sanity check, and you also need to know precisely which files are your source files and which are temporary; unfortunately, Visual Studio mixes some of them up together.
Speaking of the backup… hm, there is more to it: don't even play with the idea of doing your development without Revision Control system. If could be Open Source, free of charge, non-intrusive and extremely light weight even for an individual developer.
See this discussion:
Revision control systems, which to choose from?[
^].
—SA