If you have a program that can open the DWG file natively, then I would try coping the contents to a new DWG file, close and save the file, then run that programs REPAIR command (if it has one, you could use Autodesk TrueViews REPAIR/ Recover feature) and then try importing that new DWG file. You can try to import into Datacad any point along the process to see if Datacad can import the file, but those are the steps I normally try first.
Thanks! - Josh Do. Or do not. There is no try. Josh's Digital Downloads is come back online soon. Stay tuned. (soon is a relative term)