Just posting this in case anyone else had the same problem and finds their way here. I ran an aptitude update this morning and restarted my system (Thinkpad W520, Ubuntu 14.04). I was astonished when no GUI Login appeared (in the "Discrete" graphics BIOS Mode). After doing a bit of digging, I found that bumblebee had blacklisted the nvidia kernel module in /etc/modprobe.d/bumblebee.conf . I don't know why this started to happen, but my solution was to uninstall bumblebee:
$ sudo aptitude purge bumblebee
You may also have to reinstall the nvidia drivers:
$ sudo aptitude reinstall nvidia-___
where the `___` is the version number.
Anyways, now things work in "Discrete" graphics BIOS Mode. Haven't investigated "NVIDIA-Optimus" mode yet.