What version of AutoCAD are you using?
What version of Visual Studio are you using?
What versions of the .NET framework are loaded on your machine?
Is there an error associated with the crash?
If so, what is the error?
Have you checked the EventViewer for an error message?
Have you checked the output window in Visual Studio to see if there is some indication as to the error?
What does Helpers.GetACAD function actually do?
Do you have all appropriate references added?
Are you implementing the IExtensionApplication interface?
What is you debugging configuration?
How is the dll loaded into AutoCAD?(Netload,Script,Registry,LISP,Another Plugin)?
It would be easier to help if we could duplicate this problem, but I just don't have enough info.
You best bet for help is to ZIP up your project and post it here. At least we would be able to point you in the right direction.