Issue
My virtual machine of 64-bit Debian 7.5 (wheezy) was working in VirtualBox 4.3.12r93733 on a Windows 8.1 Pro (64-bit Operating System, x64-based processor: Intel(R) Xeon(R) CPU E5-1620 v2 @ 3.70GHz 3.69 GHz) machine (Dell Precision T3610) yesterday. But when I tried it this morning I got an error message saying: VT-x/AMD-V hardware acceleration is not available on your system. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot. I chose to continue but as promised I made it as far as choosing between system modes (regular or recovery) before the screen blacked out.
When I searched this message online I found answers saying to make sure the BIOS had virtualization enabled. My BIOS has 3 options under Virtualization Support: Virtualization, Virtualization for Direct I/O, and Trusted Execution. The first two were enabled but the last was not. (This is a work machine, so I am hesitant to load defaults without speaking to someone from IT first.)
Aside from downloading and initiating an install for Visual Studio Express 2012 (which has since been uninstalled), little has happened on this machine since the Debian virtual machine was last working. So I also investigated and uninstalled the Windows Updates from yesterday on, in case they were involved. (One in particular mentioned having to fix the BIOS.) The ones that were marked important, including the one that fixes BIOS, have been reinstalled.
At this point I started looking into VirtualBox's settings. In my online research I found several forum posts recommended going into Settings->System->Acceleration, a tab that is greyed out for me. While at Settings->System->Motherboard, I noticed my pointing device was set to USB Tablet. When I changed it to PS/2 Mouse and tried the VirtualBox again, the error message went away but the OS still does not successfully boot.
My most recent revelation happened after this: Under Settings->General->Basic, I noticed my version was set to Ubuntu (32 bit), even though I am sure it was at Debian (64 bit) yesterday. But only 32-bit OS's are options, when my machine ought to be capable of having 64-bit ones too.
My question is: What could have caused VirtualBox to lose all 64 bit options, including a working Debian (64 bit), overnight?
Solution
You probably have had Hyper-V installed and enabled.
Cross check and Disable the setting from : Control Panel >> Programs and Features >> Turn Windows features on or off
Reference : https://forums.virtualbox.org/viewtopic.php?f=6&t=57926
Answered By - user3672542 Answer Checked By - Marilyn (WPSolving Volunteer)