Setting up Mac OS X on a PC carries its own inherent risks; despite the fact that Hackintoshes are generally very stable, there's always a chance that your computer will fail to start, for whatever reason. In these cases, your best option may be to temporarily boot Mac OS X with the help of a boot USB drive. From there, you can fix whatever problem your Hackintosh is experiencing. Your Mac OS X installer USB drive can act as backup boot USB drive, but if you don't have yours anymore, you're not out of luck-- you can still create a makeshift boot USB drive on Windows instead.