I have repeated this issue multiple times.
I can confirm the workaround (Ctrl+Alt+F1, then Ctrl+Alt+F7).

I have the following HP Envy 15t with Nvidia 740M (from Costco):
http://reviews.costco.com/2070/100070028/hewlett-packard-hp-envy-15t-quad-1080p-laptop-intel-core-i7-4700mq-2-4ghz-reviews/reviews.htm

On a similar, perhaps related note, when using the touchpad, (rather than the USB mouse) I often lose responsiveness of the pointer (happens every 10-20 minutes, at least) and I have found that the same workaround (switching to the F1 virtual terminal and back) restores the use of the touchpad.  Anyone know if there a bug report for the touchpad issue already?

-----------------------
Thanks everyone.

Here's some details from my system:

(note: I did an apt-get dist-upgrade + reboot about 24 hours ago and that didn't fix anything.  I'll try again after posting this, but I wanted to capture the system as-is first.)

$ uname -r
3.13.0-24-generic


$ apt-cache policy nvidia-331
nvidia-331:
  Installed: 331.38-0ubuntu7
  Candidate: 331.38-0ubuntu7
  Version table:
 *** 331.38-0ubuntu7 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/restricted amd64 Packages
        100 /var/lib/dpkg/status


$ apt-cache policy nvidia-331-updates
nvidia-331-updates:
  Installed: (none)
  Candidate: 331.38-0ubuntu7
  Version table:
     331.38-0ubuntu7 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/restricted amd64 Packages
        100 /var/lib/dpkg/status


$ lspci | grep -i nvidia
01:00.0 3D controller: NVIDIA Corporation GK208M [GeForce GT 740M] (rev a1)


$ dpkg --get-selections | grep -v deinstall | grep -iE '(nvidia|optimus|prime|bumblebee)'
nvidia-331					install
nvidia-libopencl1-331				install
nvidia-opencl-icd-331				install
nvidia-prime					install
nvidia-settings					install
prime-indicator					install


$ dpkg --get-selections | grep -v deinstall | grep -iE '(nvidia|optimus|prime|bumblebee)' | awk '{print $1}' | xargs apt-cache policy
nvidia-331:
  Installed: 331.38-0ubuntu7
  Candidate: 331.38-0ubuntu7
  Version table:
 *** 331.38-0ubuntu7 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/restricted amd64 Packages
        100 /var/lib/dpkg/status
nvidia-libopencl1-331:
  Installed: 331.38-0ubuntu7
  Candidate: 331.38-0ubuntu7
  Version table:
 *** 331.38-0ubuntu7 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/restricted amd64 Packages
        100 /var/lib/dpkg/status
nvidia-opencl-icd-331:
  Installed: 331.38-0ubuntu7
  Candidate: 331.38-0ubuntu7
  Version table:
 *** 331.38-0ubuntu7 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/restricted amd64 Packages
        100 /var/lib/dpkg/status
nvidia-prime:
  Installed: 0.6.2
  Candidate: 0.6.2
  Version table:
 *** 0.6.2 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status
nvidia-settings:
  Installed: 331.20-0ubuntu8
  Candidate: 331.20-0ubuntu8
  Version table:
 *** 331.20-0ubuntu8 0
        500 http://us.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status
prime-indicator:
  Installed: 0.1-1+git20140109~webupd8~0
  Candidate: 0.1-1+git20140109~webupd8~0
  Version table:
 *** 0.1-1+git20140109~webupd8~0 0
        500 http://ppa.launchpad.net/nilarimogard/webupd8/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status


$ glxinfo | head -n7
name of display: :0
display: :0  screen: 0
direct rendering: Yes
server glx vendor string: NVIDIA Corporation
server glx version string: 1.4
server glx extensions:
    GLX_ARB_create_context, GLX_ARB_create_context_profile,