Ubuntu Guest freezes up
My environment is Windows 7==>Oracle Virtual Box==>Ubuntu 12.04. The whole system worked well yesterday all day. This evening I restarted Win7, opened the Virtual Box and clicked on START. Ubuntu VM showed up but my mouse cursor froze. Thus I could not move it to the VM. Somehow some changes in the VM could be initiated. If I click on Ctrl-Alt-Del no usual choice of 5 or 6 options on a blue screen shows up where I could have started the Task Manager, no, this does not get to it. Instead certain things happen in Ubuntu like the system wants me to sign in and I can do it with my keyboard, etc.
Obviously I cannot work this way. It happened 3 times already. What is going on?
Thanks, - Alex