I recently had to uninstall VirtualBox and reinstall it for various reasons that are unrelated to the purpose of this post. After reinstalling it, I noticed that the option to create a 64 bit virtual machine had disappeared, even though it had worked before.
The version of VirtualBox that I uninstalled and have reinstalled is version 4.3.2 for Windows.
Before anyone suggests to check my BIOS and make sure the virtualization technology is enabled, I assure you that it is.
Since the discovery of this issue, I have updated my BIOS, turned off virtualization, and turned it back on. I am trying to create a 64 bit Ubuntu 13.10 machine, which I had before. What is the problem, and how can I fix it?