virtualbox.org

A few of us are having problems with the above. Seems to be a problem when upgraded to Windows 10. We have 64-bit processors but can only select 32-bit versions from the drop down. We're using version 5.0.4.

Any workarounds?

FAQ: I have a 64bit host, but can't install 64bit guests.

I have the same problem.
I'm sure my CPU supports Intel visualization technologies, and it worked before I upgrade to Windows 10. My friend also gets the same problem, his CPU is the latest generation of Inter I5, which surly supports visualization.
This must be a compatible bug for Windows 10

FWIW, a CPU may very well support virtualization but it has to be enabled in the BIOS.

I am running 4 64bit guests on a Windows 10 host so it doesn't appear it "must be a compatible bug for Windows 10". You should read the entire thread. There are other reasons even if VT-x/AMD-V is enabled.

If 64bit support was previously working on the same host PC, and failed when the host was upgraded to Windows 10, then plainly some software has been installed which contends with VirtualBox for access to the virtualization features. This is dealt with in the linked FAQ.