Bug in ModBuddy when renaming files
If you right click a file in ModBuddy and rename it, the rename on disk is immediate. If you then exit the project without saving, the file name in the .sln file (or whichever one it is) is incorrect. When you re-open the mod you will be unable to build it as the file in the mod properties no longer exists on disk (it's been renamed)
The fix is either to manually edit the sln file (it's an xml file) or rename the file(s) on disk back to what they were
That's very annoying. I always have mod properties open so I can see (from the * in the tab) if the mod properties are unsaved. However, this could easily come up in crashes and everything.