Sometime, when upgrading your debian linux disto, a new kernal is installed and after reboot, the X server no more starts. This is because, now using a new kernel, the nvidia kernel module is not properly recompiled to fit the new kernel. To solve this you can manually remove and reinstall the related package to force a fresh install that will fit your new kernel.

Read more Repaire a broken nvidia config after debian upgrade

Just open shell as root and enter the two following commands :

# aptitude update
# aptitude upgrade

Read more Update debian in two command lines