What I discovered through reading this forum and a few bug notes is that VT-x may have become disabled. I have a Dell 745, BIOS version 2.3.1 (factory installed). I went into the BIOS (the PC's BIOS, not the virtual machines BIOS) and found that there is a CPU Virtualization setting. It is: Performance > Virtualization and the setting for the CPU is simply on or off - the default being OFF. After turning this on, the problem was corrected as I have *always* had VT-x enabled for my workstations.
I have never muddled with the BIOS settings, and so I have a theory for what may have happened. It could be that the setting was always turned off - but the kernel that I was provided by Ubuntu simply ignored it. A newer, smarter kernel may have recognized this setting and used it, which disabled VT-x to Ubuntu.
Just a theory - but this cost me a fair bit of time to figure out and so I am hoping someone else finds this information useful.
-Ray