Sorry to hear that - (I hope you had some earlier version of the level saved that you can go back to!)
This is the first time I hear of this problem, and I've never encountered it myself.
The first thing I would do would be to restart my computer - start it fresh again without any other programs open (In case it is a lack of RAM memory) and see if that makes a difference.
If it still doesn't work, my last resort would be to try to transfer the level to another more powerful computer and see if it loads on that one.