Depending on your plan level you may or may not be able to set this, but there is a session limit which defaults to 720 minutes. That is 12 hours. So that probably what happened. The user will not have lost "all her work" just whatever echoed she was working on, although I guess she could have been in the middle of a giant grid edi update.
Generally when I get hit by that ( which is more often than I care to admit) I think to myself, man it's time to get away for the keyboard for a few minutes as I've been at it for 12 hours now.