Have you tried reflashing the BIOS?
Also disconnect all devices that the system dosent need to boot, so no jeyboard nor mouse nor any HDD nor ODD, then see what happens when it boots. IF it boots normally and gets to the point where it say that there is no bootable device then start adding one device at a time, and then booting to see if it gets to teh same place, at some point you will add a device and it will get stuck. That will be your issue.
If you take everything off and it still gets stuck you need to start changing graphics and memory around