When you boot, the bios runs POST (Power On Self Test). The boot hangs when one or more of the items tested (i.e. HDD availability, RAM functionality, etc.) fails. When you reboot, POST is skipped, so that you may successfully boot even though something is not working properly. ISS is right in suggesting you start to check your hardware and drivers, BIOS, etc.. You are going to have to go through a process of elimination.
Suggest you turn on your speakers and cold boot to see if your BIOS give you a beep code which you can use to help diagnose the problem.