virtualbox.org

What is the recommended process when a guest OS (F11 in my case) crashes? In that state, "reset" and "ACPI shutdown" seems to refuse to work. HItting 'Close" in that situation generally seems to corrupt the whole snapshot tree. What I _want_ is a way to tell vbox "Please completely throw away everything about the current state, and revert 100% to the previous snapshot". Is there a foolproof way to do this? I couldn't find anything about it in the manual or forums (but I may not have thought of the right search keys for the latter). Of course it needs to be something that can be done while the host still thinks the guest is running...
TIA
Bob