I just stated a project in Unity 3 and then realized I couldn’t publish to the web. When I brought it back into Unity 2, the entire scene I was working on is gone. Any way to get it back, or do I need to start over from scratch?
restore from backup is the only way to do that
It’s not a lost or damaged file problem. It’s a problem with Unity 2 not recognizing Unity 3 projects. But it’s not that simple. I had an earlier version of the version 3 project I had saved, and it works in version 2. So I only lost about 75% of it, rather than everything. I guess some things are compatible between versions, and some things (like whatever I added since the backup) are not.
By “restore from backup”, he meant (I presume) restoring from a backup from before upgrading to 3.0. And yeah, Unity 2 can’t read U3 files.
(Nor would I honestly expect it to. Them’s the breaks when you want to use beta software.)
You might… MIGHT… have better luck if you export a package from U3 and import into U2. Long shot.
Unity 3 projects are not compatible with 2.x unfortunately. And packages will not work either. We had to break some backwards compatibility for a major upgrade like this.
I welcome this move actually.
While it hit me too on the iphone side I prefer having an opted and future oriented thing instead of a legacy mess 
I did a little test project to see if I could go back and forth between the two versions, and it worked. Which is why I started the real project in version 3. But I guess I added some stuff in that project that made it no longer compatible with version 2.
Now I know there’s no going back. Only lost about half a day. Much less than I initially feared. And in the process of redoing the project, I fixed a nagging bug that was driving me crazy. So I’m a happy camper!
I think it was nothing that you added but beta 4+
I know that beta 2 or alike was able to move back to iphone 1.7 but with the change of mono, the enhanced texture settings etc it was likely getting a nightmare to do it without scrapping the old format