VirtualBox is able to host 64bit guest OS on Windows. That’s useful for testing various platforms.
The trouble begins when you turn on Hyper-V Windows Feature. It might happen by installing some software or update. When Hyper-V is enabled then it’s not possible to create new guests with 64bit OS and it’s not possible to boot anything with 64bit kernel created before this feature was turned on.
In order to make 64bit guests working again you’ll need to turn off Hyper-V feature.
Search for “windows feature” and open “Turn Windows features on or off”
Search for “Hyper-V” in Windows features and uncheck it.
Click OK and reboot the machine. 64bit OS guests start working again.