You have come across a rare problem. For some unknown reason, your ORM file has become corrupted, If you can reproduce this corruption, we would like to know about the reproduction scenarion because maybe we can fix the cause of the problem. However, in most cases customers can't reproduce the problem.
You have two recovery options: (1) If you are very lucky, you may be able to inspect and edit the ORM file, and get it to work again. (2) However, the typical experience is that the custoner goes back to a backup file (with a .BAK extension) and then recovers from there.