Such occurences are something everyone experiences, unfortunately.
The key question is whether the error is repeated if you open up the same file again. Usually, for me, this is not the case. If the same problem repeats with the same file, that points to likely corruption of the file. If so, you might need to investigate what happened to that file, whether you have older versions of the file before the condition happened, etc.
What I've learned from this and every other CAD program I've used in the past is to save often, save after any complicated work, save if I'm leaving my desk for a while. That way if I have a fatal error occur I minimize the amount of work I lose.
