Let me start by saying that I am not an Ubuntu user (I happen to run Fedora myself), but I just spent the better part of an evening resolving what appears to be an identical problem, so I signed up for an account in the hope that my findings can help you out.
My guess is that your atp-get included an updated xorg-x11-server-Xorg package, as two security updates have been released for it over the last two weeks. My problems started after I ran "yum update" in Fedora, which basically does the same as an apt-get.
As with any good problem, that is only part of it The root cause is the fact that the xorg-x11-server-Xorg update replaced libglx.so, and this is something your NVIDIA driver didn't take kindly to (mine sure didn't!), as it expected to find its own copy which it put there when the driver was was installed.
Fortunately NVIDIA has released a driver update (169.09) which takes away that problem. From its release highlights:
Fixed a bug that caused the X driver to crash if the X.Org GLX extension module was loaded intead of NVIDIAs.
Installing the newer NVIDIA driver should fix the problem for two reasons:
1. The core NVIDIA driver can deal with a replaced libglx.so module.
2. Installing the NVIDIA driver causes the replaced module to be re-replaced with an NVIDIA specific version.
Hope this helps!