Most likely the computer does not have an operating system installed on it. That's what that questionmark - happy folder icon indicates.
Is it an iMac computer? If so, boot with the option key held down -- this will present you with a list of bootable volumes connected to the computer. If no volumes show up, there is no bootable volume for the computer to boot from and you must install an operating system, either from Mac OS 9 or Mac OS X install CDs/DVDs. If you do not have a licensed copy of either of these, you must purchase them.