As usual, when I installed Unity 4 (free version) and tried to open one of the projects I made in Unity 3.5, The project was upgraded. Then when I tried to open when of the scenes in it, Unity always crashed. However the other scenes containing only GUI elements don’t cause crash. I’ll appreciate if you can help me out since I don’t want to downgrade unity or remake the scene.
Thank you very much for any suggestions!
If the editor is freezing when you hit the “Play” button on your scene I’ve had the same problem and managed to get around it.
I opened the scene in the editor and started by deactivating all objects in the Hierarchy. Then I activated them one after another and tested to hit play.
When the game finally froze I then knew what object(s) were causing it and removed them from the scene and reimported them.
But in case your problem is about loading the scene into the editor itself I’m not sure what it could be, but I saw this topic earlier:
http://forum.unity3d.com/threads/162326-When-Project-Crashes-On-Scene-Something-I-found-Useful
Good luck and I hope you get it working properly again! 
Thank you very much Kona! I The issue in that thread is very close to my issue, but still different. I can always open the project but always experience crash while trying to load the scenes.
The solution you came up to solve the issue you experienced is smart. It may be very helpful if I can load the scene successfully.
Still seeking a solution of my issue…
I am having this problem as well. After upgrading the project for Unity 4, some of my scenes crash the editor when I try to open them. Did you ever find a solution to this?