Textures and Objects unassigned since Unity 4

I updated Unity 3.x to Unity 4 yesterday and everything worked fine. I started the programm today to continue working and as it finished loading everything was missing. Well, everything was missing in the Editor-Window. As I looked into the Scene Object-Manager I realized that everything was how I saved it. As I clicked on the Objects I saw that EVERYTHING was unassigned. Every Script was missing, every texture was not assigned to a material which itself wasn't assigned to the Object. In other words: I had no textures or meshs that could be seen ingame.

Does anyone know how to get everyting assigned again? Like a backup-loading? Because assigning by hand is very hard.