Possibly, you experienced a version of this:
The "good" news is that the transaction log for the course may still contain the student answers. You can access the transaction log in /opt/webwork/webwork2/logs/. It is helpful to apply grep or sed or something to throw out all lines except the ones wfor the student in question, for the quiz in question.
If there is any way to get hold of the browser console log from when this happens, that could be helpful. Or of course, if there is any reliable way to reproduce it.