Versioning of models was introduced with the Digital Object Repository in version 12 of Datawatch's server product Automator. Until then, it's just good policy to store the models in a location that IT back up for you daily, so if somethings goes wrong, you can ask for a restore of the xmod file you need.
If that's not possible, then you can try two things - to get Monarch Utility to "copy models from one place to another" - this might fix the issue. Another is to create a new model and pull in all the required objects from the defunct one - this also may work.
What's best or easiest depends on the model - if you can share it by email I'll happily advise.