Get a bootable cd (Fedora install cd, cd-based distro etc) and boot it. Then look into /var/log/messages file on your disk (you may need to mount the partition - if you don't know how to do it, ask). At the end of the file there's your last boot. Browse the messages, see what fails first. That's the main reason. When you have first messages you may already know what happened and how to fix it. If not - post them.
The ones in your posts are OK (well, if error messages may be OK...), but I think that there was something earlier.