Error message "Not in a hypervisor partition"

[KJB]

For a few years I have been running an old version of VirtualBox (I think 5.2.8) (EDIT I mean Five Dot Two Dot Eight) on my Windows 10 64-bit pc, without problems. Yes I have had notifications to upgrade, and ignored them.
This weekend, my Windows 10 installation was updated. I couldn't start the Virtual Box manager anymore.
I downloaded the VirtualBox 6.1.16 installation package and the corresponding extension pack, and installed them over the old installation seemingly without trouble.
My old virtual machine is visible in VirtualBox Manager, in the Powered Off state.
If I doubleclick it to open, I get the following error message:

Failed to open a session for the virtual machine Windows XP
Details:
Not in a hypervisor partition (HVP=0) (VERR_NEM_NOT_AVAILABLE).
VT-x is disabled in the BIOS for all CPU modes (VERR_VMX_MSR_ALL_VMX_DISABLED).
Result Code:
E_FAIL (0x80004005)
Component:
ConsoleWrap
Interface:
IConsole {872da645-4a9b-1727-bee2-5585105b9eed}

I am not very computer-knowledgeable. Does this error message refer to a hypervisor partition (whatever that is) and the BIOS on the host machine?
More importantly, how do I solve this issue?

[scottgus1]

Either your PC BIOS does not have the "Virtualization Technology" (VT-x/AMD-V/SVM) turned on, or something on the PC, like Hyper-V or 3rd-party antivirus, is interfering. Go through every part of
I have a 64bit host, but can't install 64bit guests

[KJB]

Is that applicable even though the guest is Windows XP and so not 64-bit?
And does it make sense if it worked until a few days ago? I mean, could the Windows 10 update on the host have messed with the BIOS?

[scottgus1]

even though the guest is Windows XP and so not 64-bit?

Yes, because it's a host OS issue that is causing the error.

could the Windows 10 update on the host have messed with the BIOS?

Not the BIOS, no. But the other stuff in the rest of the tutorial, yes.

[KJB]

Thanks for the superfast responses.
I will try later tonight, and report back with either success (hopefully!) or, well, whatever pops up.

[KJB]

Success!

In the BIOS, I changed Virtualisation (that is VT-x) from Disabled to Enabled. After rebooting, everything worked as usual.
So I needed to be in the BIOS after all. I assume that virtualisation was disabled all the time while VirtualBox 5.x worked OK. But now with VirtualBox 6.1.xx, it needs this VT-x type of virtualisation to be enabled.

Thanks again for your help, scottgus1.

[scottgus1]

virtualisation was disabled all the time while VirtualBox 5.x worked OK. But now with VirtualBox 6.1.xx, it needs this VT-x type of virtualisation to be enabled.

Correct. With 6.0.x & earlier, "Virtualization technology" (VT-x, AMD-V) could be skipped, with certain limitations. Starting in 6.1.x it is required for all VMs, even the ones that ran before.