My Ubuntu session running in virtual box sometimes freezes and becomes totally unresponsive when windows goes into sleep. There is no option other than to "power off the machine" and then restart fresh again. Can anyone facing similar issues suggest solution? Already tried options which came up from google search but no breakthrough.

Crude but effective: I had a similar issue and disabled hibernate and sleep. It is tactical but allowed me to complete a design over a few days. (Not a good longer term solution.)

@Andrew Wright What do you do in case not want to close current ubuntu session on virtual box and want to start from same state next day ? Once laptop screen is closed it will go into sleep or hibernate causing ubuntu session to freeze when laptop is opened next time. Can you suggest any solution or workaround.