The question mark means your computer can't find Mac OS. Try holding down the option/alt key as you turn it on. It should show you all the disks you can boot from. If you can find a "Macintosh HD" or similar in there, select it with the arrow keys and hit Return to start up.

If that works, you probably just have the startup disk set wrong. Pull down the Apple menu, hit System Preferences and go into Startup Disk. Select your hard disk in the list that comes up and close it.

Hope this helps.