It likely isn't a matter of an open connection in code that didn't get closed. It is likely that something like a network disturbance or someone had to shut down their computer with a hard reset or exited Access by using Task Manager to kill the process. It is hard to tell exactly. But if you haven't had this before, I would wait to find out if it really is a problem before trying to solve the problem. I know it can be a pain if it is a problem and it locks up. But going off and trying to find a problem based on a single event is potentially tying up resources that could be put to better use if there is no evidence that a real problem exists.
That's just my take on it.