OS X Lion: If you can’t start up after resetting PRAM
If you have a Redundant Array of Independent Disks (RAID) scheme set up, your computer may not start up if you reset parameter random-access memory (PRAM) when you restart.
If this happens, hold down the Option key while you restart your computer. Then select your startup disk when you see the prompt. After you successfully start up your computer, choose Apple menu > System Preferences, click Startup Disk, and select your startup disk.