having trouble starting up

Hi! It means the computer can't find a device to boot from. It could mean your hard drive is bad, or that the the drive is good but that the OS has become corrupted. The first place to start is to boot from the restore DVD (put it in the drive and boot while holding down the "C" key) and do a restore without wiping the drive in order to see if the OS can be repaired. If during this process the hard drive does not show up, it may be bad. Also, when you are on the screen that lets you select the hard drive to install the OS on, you can pull down the menu from the top and choose Disk Utility, and then run a check on your drive, which may let you know if your drive has a hardware issue. If you drive is not visible at all, it is sometimes still possible to see it in Disk Utility, format it from there, at which point it will become visible in the installer. Good luck, and let me know what you find!