Check to make sure that the part hasn't gone into hiding. Is the part or the body hidden?
What do you mean that zoom to fit doesn't seem to work?
If you can't seem to get the model working correctly, try converting it again. Check the part to make sure that it imported correctly and doesn't have any errors before you make any changes.
Can you please attach the model ???
It may have created something out in space so when you fit to screne its doing just that.
I have created reference points to intersections and had this happen also.