Re: why does this happen in my perfect OS?
First make sure that your monitor is connected to the nvidia card, not the onboard (seems stupid but i have seen this mistake before) and check the BIOS to see if there are some options regarding add-on cards initialization. You can disregard this if you know what are you doing/the card worked before in this setup.
You must uninstall all repository packages related to nvidia before you install the nvidia-provided driver and rename your xorg.conf. Also make sure you blacklist the nouveau driver (this one loads automatically and it conflicts with the nvidia module) and restart.
Probably the GUI wont load at all after reboot - just press ctrl+alt+f1, log in the text terminal, stop the gdm service and install the nvidia driver. Then you can start the gdm service.
Note that you must have the kernel headers installed (but i think that in Ubuntu those are installed by default).
Also, if you have installer errors, chack the sticky about the log files you have to provide in order to be helped (i think the nvidia installer creates an nvidia-installer.log file, look for it and attach it to your message).