[ubuntu] VirtualBox seems to have just deleted itself ??

I have Virtualbox installed on my Linux workstation now for at least half a year, running Windows 7 as a guest system.
I had it running this morning, then shut it down (selecting "save the active state" for faster reboot).

Now this afternoon, i wanted to work on some stuff under the windows environment and wanted to fire Virtualbox back up.
However now I got this strange warning:

Failed to execute command "VirtualBox %U".
Failed to execute child process
"VirtualBox" (No such file or directory)

Virtualbox is also not recognized as installed in the Ubuntu Software center. The commands "vitrualbox" and "VBox" are now missing from my /usr/bin/-folder. However in my /usr/bin/ folder I DO find "VBoxClient", "VBoxControl" and "VBoxService".
I certainly did not delete or uninstall them or use anything requiring sudo, as far as I can remember...

The only thing that may be somewhat related is the Software centers request to do a "partial upgrade" today, which lead to an error message about some lib3xml stuff not being able to be updated. I can't remember what exactly the issue was, but virtuabox was still running at the time and had no problems.

I can still find the folder for the virtualbox VM in my home folder, so I have great hopes of maybe just reinstalling virtualbox and trying to reuse that same VM. But this whole thing DOES make me kind of nervous...

Any Ideas on what may have happened?

BTW: I am running Ubuntu 14.04.5 LTS with the xfce4 desktop.

EDIT:
Here's the first 15 lins of the last logfile I found in my VM-folder:

VirtualBox VM 4.3.36_Ubuntu r105129 linux.amd64 (Jan 27 2016 12:09:06) release log
00:00:00.454423 Log opened 2016-08-29T07:21:51.813706000Z
00:00:00.454425 Build Type: release
00:00:00.454428 OS Product: Linux
00:00:00.454429 OS Release: 3.13.0-93-generic
00:00:00.454430 OS Version: #140-Ubuntu SMP Mon Jul 18 21:21:05 UTC 2016
00:00:00.454457 DMI Product Name: Precision T5610
00:00:00.454463 DMI Product Version: 01
00:00:00.454555 Host RAM: 80502MB total, 56316MB available
00:00:00.454558 Executable: /usr/lib/virtualbox/VirtualBox
00:00:00.454559 Process ID: 854
00:00:00.454560 Package type: LINUX_64BITS_GENERIC (OSE)
00:00:00.519683 Installed Extension Packs:
00:00:00.519700 VNC (Version: 4.3.36 r105129; VRDE Module: VBoxVNC)
00:00:00.528450 UIMediumEnumerator: Medium-enumeration finished!