So I encountered this issue when opening up my Unity 5 project. All of a sudden my materials and prefabs are no longer recognized by Unity as being a file of that type, and as such I can’t edit them in the inspector or continue to use them in my project. The existing GameObjects in my scene still have the materials and the prefabs still appear blue as if they are connected, but I can’t use them anymore or make any changes to them. Not really sure what’s going on here, it probably has something to do with my storing the project in Dropbox, but I have been working on projects like this using Dropbox on a dozen different computers and haven’t encountered this issue before. I also recently worked on the project on a Windows 8 machine when I had previously been using OSX and Windows 7 machines, not sure if that could be related.