Android on virtualBox gets stuck after idle
I was able to get back to the screen by clicking ACPI shutdown menu(HOST + H)
It is pretty simple. Just hold ESC + MENU (A key between right-alt and right-ctrl) and the machine will wake up.
I had this problem today and couldn't solve with any keys provided by the documents.
Android will resume if you try to close the VirtualBox window, and when asked, select to send shutdown signal instead of forced shutdown.