Today I experienced that VirtualBox would only run 32-bit virtual machines on a laptop.
That was a problem for me, because I wanted to do try some things on a 64-bit Ubuntu Server.
I decided to reboot and check the BIOS virtualization settings.
There I discovered that both “Intel Virtualization Technology” and “Intel VT-d Feature” were disabled.
I enabled the settings, started Windows and after that 64-bit virtual machines could run again in VirtualBox.
I was convinced that the settings were enabled before, but I suspect that BIOS settings were reset the last time I updated to a new BIOS version.
If you can’t run 64-bit virtual machines in VirtualBox, check and adjust your virtualization settings in BIOS/UEFI.