Ubuntu – Ubuntu 13.04 won’t shutdown or restart (Just a black screen)

I have looked all over the Internet trying to find a fix, but I can't seem to find one.
I have a custom built PC running an AMD processor, Nvidia graphics, 16GB of ram and a 1TB HDD for Ubuntu. I am not trying to dual boot, I just have Ubuntu 13.04 on the hard drive nothing else.

Every time I press 'Shutdown' or 'Restart', it goes through its normal shutting down thing; but, after that, there is just a black screen and nothing more. Nothing happens, it's just black. The only way for me to turn off Ubuntu at the moment is to press shutdown, wait for the screen to go dark, and then force shutdown it by pressing the physical shutdown button on the PC.

Could someone please tell me what is wrong and how I can get Ubuntu to work correctly? (Never had this problem when I was dual booting on my old HDD.)

P.S. I am able to log out.

After running Sudo shutdown it gave me an error this is the error:

'Modem-manger[5062]: Could not get the system bus. Make sure the message bus daemon is running! Message: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory
modem-manger[5078]: ModeUbuntu 13.04rsion 0.6.0.0) starting…
modem-manger[5078]: Could not get. . . . m bus. Make sure the message [FAIL] e
* Will now switch to single-user modenect to socket /var/run/dbus/system_bus_so
root@mauritz-ubuntu:~# irectory'

And then just a ton of OK checking's,
To me it seems like much of the text that it is displaying is overlapping on other text, so thats why it seems really weird.

Second error (running sudo shutdown as root):
"sudo shutdown now
*Disabling power management… [OK]
Saned disabled; edit /etc/default/saned
Speech-dispatcher disabled; edit /etc/default/speech-dispatcher
* Asking all remaining processes to terminate… [OK]
* Killing all remaining processes… [FAIL]
* Will now switch to singel-user mode"