I'm new to ubunutu and are experiencing some problems with using the newest nVidia driver 352.63 from nvidia-352 (Proprietary, Tested).
After applying it (In software & updates -> additional drivers) and rebooted my system i got a black screen when the login was supposed to appear. I did find a work-around, by closing the lid so my system will hibernate, and when it wake it up i can see the login
I'm running Ubuntu 14.04 LTS, with a i7-3630QM, and a Geforce GT 640M LE
I like to add that i also experienced problems with the nVidia driver on Windows