Snapshots can be a pain, but they don't save you from errors like these. You can go back to a previous version without issues. The problem here is that you had VT-x/AMD-V available, and something on your system changed, so it's now in use by something else, or it's disabled. Check your BIOS settings, maybe it reset itself and turned it off. Also check the VM settings, make sure hardware acceleration is enabled. Next is checking the Host to see if you have other virtualisation software installed that locks the CPU, like KVM. The
VirtualBox FAQ has some tips on how to find and possibly fix the error.
If all fails, check for a BIOS update and install that, it might be a bug in the BIOS that's now triggered by VB. They put in various checks for BIOS bugs and such to make sure the VM runs stable. This has happened in the past before, where a new release had some BIOS checks and caused all sorts of issues for those with buggy BIOSes. They had to update their BIOS to fix it.
Should I have created a snapshot prior to installing the VB update?