为什么我所有的引用在。net类库中都有黄色的感叹号呢?

I've been developing a class library for quite sometime and all of the sudden, sometime last week, I opened my project and all of my references have yellow exclamation points on them now (System.dll, System.Drawing.dll, etc..). I've tried removing the references and re-adding them to fix any broken reference paths, but they continue to show yellow exclamation points on them. I've been developing a class library for quite