Problems installing linux

Most updates that lead to a blank screen have something to do with a graphics driver problem. You can stop at the grub screen, add a 3 to the kernel line, boot up in init 3 and check /var/log/Xorg.0.log to see if errors come up while initializing graphics.
Won't boot covers a lot of territory. Does it get to the grub screen and not to login? Does it get to login but not desktop?
I would first look at the boot scroll. If it's covered with graphics, hit escape during boot. See if scroll stops somewhere during boot, and, investigate that line.
Checking /var/log/messages helps.
Checking /var/log/dmesg also helps.
But, like I said, "does not boot" covers a lot of ground.

The first thing you do it attempt to pinpoint where the failure is happening. Does it error out at the bootloader, if so what error messages are presented? if it is during the boot process where did it freeze and have you checked the syslog?

One nice thing about Debian, and I'm hoping Ubuntu also, is bootlogd. If it's enabled, it will write the whole boot scroll to the boot.log file. When you want to troubleshoot, that has come in handy for me. Fedora doesn't have a very verbose boot.log.