I had exactly the same problem recently, as did at least 2 other members here. See the following threads for details:
(refer to my first post in this thread)
At this stage, a solution hasn't been forthcoming. It's very weird that the problem has arisen only recently. Scratching my head trying to think of what has changed - was it a Mountain Lion update perhaps, or installation of a 3rd party app?
PS I'm off to bed now so will respond to further posts later tomorrow.
Message was edited by: John Cogdell - added PS
John - Thanks for putting my mind to ease that this isnt just a one time thing. As evil as it may sound, im just glad im not the only one experiencing this!
I suppose all i can really do is wait until Apple fixes it, ill put in some feed to apple myself at some point, thanks again for the help.
Oh and if i may ask, this Time Machine thing (in new to Mac), is it something i should be running all the time? And also, is it possible to attempt to repair a file?
If you have Time Machine running on a backup external hard disk or on a Time Capsule, you should have it on at all times. You may have picked up in my comments in the linked threads that I had mine turned off while editing the "lost" project. The backup disk was full and Time Machine was refusing to add further backups. This is unusual in that Time Machine normally offers to delete older backups in order to fit the latest one on the disk. This allows you to keep using the same disk for backups, as long as you don't mind losing some of the earlier backups (mine went back to about a year ago). Anyway, I turned it off and hadn't got around to connecting a fresh disk. That's now been rectified!
Here's a link to an Apple Support article entitled "Mac Basics: Time Machine":
Let's hope we get a solution to this vexing problem of the "Disappearing Project". When I get a chance, I will be submitting feedback to Apple. The more that do this, the better chance we have of getting it looked at by Apple.
It's sad that it apparently has not been fixed. Luckily, I lost only the one project. I haven't used iMovie much since then (mainly been playing around with Final Cut Pro X).
There are numerous reports about this issue on the forum. I think that something happened in late 2012 that may have caused iMovie to misbehave in this way - possibly an iMovie or Mountain Lion update.
What version of iMovie and Mac OS X are you running?
I'm on OSX 10.8.4 on MBPR, iMovie 11 9.0.9.(1795).
The funny thing is that I was evaluating iMovie vs. Microsoft DVD maker for a small group of family who were interested to see if they should buy OSX instead of Windows 8 for several new computers (students and family computers mostly).
I had made a movie last year with DVD maker in about 60 minutes and they loved it. This was my first use of iMovie ... not sure yet if there will be a second use.