I don't have enough reputation to comment, so i have to type here in answers :(
I would start by using Grep to search /var/log/kern.log and /var/log/messages. You can search for the time when the system froze, and see what logs where generated before that. If your certain it was virtual box, you can search for that and see if you can dig any more information on it. There are also log files that are generated within Virtual box that contains information regarding BOTH the host and the vm. That might be a good place to look also if you believe it was the issue.
Check out https://blogs.oracle.com/scoter/virtualbox-log-files-v2 for how to find those logs.
Never-the-less, i would recommend KVM instead of V-Box. Oracle is not updating it as often, and the performance gains of KVM is starting to take a pretty far leap ahead in performance where it no longer makes sense to use V-Box, other than the awesome GUI. Not to mention they currently have some killer security holes known right now, and oracle has yet to update them after some time. Debian even dropped V-box temporarily from its repo because of the lack of responsibility of oracle updating the code.