Press delete or f1 when the asus pic comes up. The asus logo is hiding the your POST messages, (PowerOnSelfTest), its commonly covered by manufacturers logos. When you hit del or f1 it will enter the bios setup mode. Enable boot time diagnostics, or disable boot logo or whatever they are calling it on that particular board (bios options differ depending depending on board and manufacturer), should be in the boot options tab. Now next time you reboot you'll be able to actually watch it do is mem tests, detect drives, etc, all the things it does every time anyways but that its currently hiding from you behind a manufacturers logo. By watching this you can see at what point it binds up if it does it again and will go a long way to pinpointing the problem.

Could be alot of things, failing to detect a hard drive, disk drive, memory stick, corrupt bios settings, etc etc. See what stage of the POST its stalling on if it does it again, and if it's correctly identifying your processor, ram, drives, etc. If it's stalling during the POST (while the asus logo is up) then its either a hardware problem, bios corruption, mobo battery died and reset your bios settings, or something along those lines. It's not installed software or drivers, software and driver conflicts happen after POST, not during, usually about the time the windows logo appears.