More information will be needed. Such as: C3D version and what service packs have been installed. Operating system, computer specs, what were you doing when you got that error, did you submit the CER (Customer Error Report) to Autodesk, has this occured more than once, in just one drawing or multiple drawings.
Still not enough information posted for this issue to provide much meaningful feedback.
Can you open the file or is it crashing prior to fully opening?
If it won't open try opening the file by going to the application menu>Drawing Utilities>Recover.
Additionally if the drawing has xrefs attached to it run a recover on all xrefs as well.
On occasion the file causing the crash isn't necessarly the one you opened but a referenced file.
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