if Virtualbox was working fine and you updated Virtualbox lately it could be a bad update
install or something is wrong with the version of Virtualbox you are using and you found
a bug
i'v had a bad version update of Virtualbox that act's like this and would not let me install
or run a 64bit OS it would only run a 32bit OS
so i went back to the version of Virtualbox before the update and it worked i could use my
64bit OS again so i let a few updates pass before i tryed again and it worked that new
version was ok
i see someone else saying something like this on the Virtualbox forums
Problem using virtualbox
https://forums.virtualbox.org/viewtopic.php?f=6&t=60866
an easy way to check if AMD-v is turned in the BIOS is try this
Microsoft® Hardware-Assisted Virtualization Detection Tool
The Hardware-assisted virtualization (HAV) detection tool checks
if the computer processor supports HAV and if this setting is enabled.
http://www.microsoft.com/en-us/download/details.aspx?id=592
James