64 bit and intel virtuel settings gone

[FrankJor]

I've long been running virtualbox with no problem. Had a few 64 Bit windows VMs and it was going great. Then one day after a few days of not using them, they all show up as 32 Bit and I am no longer able to select 64 bit when creating VMs. I can't start the VMs of course, as they have 64 bit windows installed. The CPU acceleration tab is greyed out and no way to select it.

I could say, oh well the system doesn't support it. But it does, as I've been running with these VMs with no problem. There's no bios option to change, so it is not there either and there has never been a bios update for my system, so it is not that either.

The system is a clevo laptop P370EM3 and the CPU is an i7-3720QM (which supports intel virtualisation)

So how do I fix this?

[mpack]

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

Minimum information needed for assistance.

[socratis]

Did you enable Hyper-V in your host? Or installed some other program(s) that may get an exclusive hold of the host's VT-x capabilities?

[FrankJor]

The host is Windows 8.1, so likely something has taken over VT-x, though is there any way to see what has? Does the virtualbox logs tell you this?

mpack: Yes, I've read the minimum requirements, but most of it doesn't apply to the situation at all, so there wouldn't be a point in including it. Other than I forgot to mention what OS the host was running.

[socratis]

You can check if Hyper-V is enabled by going to: Control Panel, Programs and Features, Turn Windows features on or off, Hyper-V. I don't know how to check for other programs or actually which programs. I've heard that Window Phone Emulator also does that, as well as some antivirus programs.

You said you can't start the VM. What's the exact error message? And yes, the logs will tell us whether or not VT-x is available.

[mpack]

All the log will show is that VT-x locked. No way to know who locked it. For the moment, the VT-x resource is not managed the host OS, so there's no API to query.

Any log will give us many useful details about host and guest which would otherwise have to come from Q&A with you, and take considerably longer, and most likely be wrong.