Intel CPUs are designed to allow virtualization to be disabled and unable to be turned back on until the next physical power cycle, likely for security reasons (i.e. a bootkit could theoretically install a malicious hypervisor). I'm not sure of the exact reason why some BIOSes don't allow virtualization to be enabled.
You will need to upgrade your BIOS to a version that allows you to change the setting if the manufacturer has chosen to do this, or scour the Internet for a hacked BIOS, if it exists, that is modified to allow you change this setting. You will have to do searching on your own for such a BIOS as I've never went that route with any system I have.