If you can't restart your computer under Ubuntu/Linux Mint and the screen turns black with no response after clicking the Restart button, you can follow this tutorial which may solve your reboot issues. This problem is common for owners of laptops or netbooks due to hardware/BIOS problems.
Open the terminal and edit this file under Ubuntu 12.10/12.04/Linux Mint 13:
sudo gedit /etc/default/grub
For Linux Mint, run this command:
sudo pluma /etc/default/grub
Find now this line:
Replace it with this one:
Save your file and close, then update Grub with this command:
Reboot your system, then try to restart your system to see if it works or not. If this is not working for you, repeat the same steps and try one of these lines instead:
If you have more solutions for reboot issues, please mention them in the comment form listed below.