Have you added any new hardware lately? Changed any of the Bios settings?
If you added any new hardware (Printer, video card, sound card) uninstall it and try to boot. Boot your computer with no deviced installed and see if this clears the boot sequence. If it does, then plug in one device at a time until your computer fails to boot. If this is the case, you now know what device is the problem.
If your computer still freezes with no devices installed, then try to reset your bios settings to default. make sure to save your settings before exiting.
If this still does not work, then you can try to flash your bios with an updated version for your mobo. If this does not work, then you prob have a bad mobo.
Good luck!