Are you actually able to login and see a desktop, or does it hang at the login screen before entering details, or after submitting the password? Next thing to try is hit Ctrl+Alt+F1 at the graphical login screen, before entering username/password. That'll bring up a console (black screen, white prompt) and
you should be able to login that way and run CLI commands. You can then read any kernel boot alerts using this command at the prompt (use the up/down arrow keys to scroll through it, and Q to quit):
Unfortunately all Linux distros can be a bit buggy with some graphics
cards, sound
cards etc., mostly due to the vendor's reluctance to write drivers for Linux or open-source their code. There are often alternatives which can be used, but we first need to find what is the actual problem!
Oh, and you might want to use "sudo reboot" or "sudo shutdown -P now" to reboot or shutdown from the command line.
[ November 29, 2008: Message edited by: Charles Lyons ]