The question mark means Where is the Operating System? If the computer boots and the hard drive has problems you will get this symbol and it usually but not always means a physical problem with your hard drive. First thing to do is follow the procedure in the previous page It Starts Up But Runs Really Badly and boot the computer from an install disk. When you open Disk Utility if instead of a hard drive icon on the left you just get an empty window then you now your hard drive has major problems and it is at this point you rejoice as you remember that Time Machine has been backing up your computer every hour, hasn’t it?! If so it is an inconvenience rather than a major catastrophe as new hadr drives are quite inexpensive. You have a new one fitted, boot off your install disk and from the Utilities menu choose Restore From Time Machine Backup. A couple of hours later your computer is back as it was before the problem.
Sometimes it is some other hardware problem that is causing the issue such as a logic board (mother board) which needs attention and will mean a trip to your local Apple Mac repair shop. here in Byron Shire ours is Lightforce.