Intel VT-x disappeared after upgrading to Windows 8

Had same issue. I enabled the Hyper-V to test the built-in virtualization in Windows 8 and that caused the OS to report Vt-x incorrectly.

I disabled Hyper-V from the Windows features and now VT-x is enabled again.

Go to Control Panel → Programs and Features → Turn Windows features On or Off and then uncheck Hyper-V.