What model iMac do you have? What version of Mac OS X are you running?
The 'interrogation' or flashing question mark indicates that your iMac can no longer find a bootable volume. It could indicate a hard drive problem, data corruption, erased hard drive, or a parameter issue preventing the system from looking in the right place.
I suggest first trying to reset the parameter RAM by pressing and holding down the Command-Option-P-R keys as soon as you power on the computer. Hold the keys until the computer restarts and you hear the startup sound for the second time. Then wait to see if your computer boots.
If the issue persists then you can use the Disk Utility application to verify the hard drive. Depending on whether you are running Mac OS X Lion or an earlier version of the OS changes how you access Disk Utility when you don't have a bootable volume. If you are running Lion then you would simply hold down Command-R when you power on the system. Once the Lion Recovery partition boots you can run the Disk Utility application listed in the menu to verify the hard drive and reinstall Lion if needed. If the hard drive is bad then you can try using Command-option-R to use the Internet Recovery method.
If you are using an OS prior to Lion then you will need to boot from the Install DVD that came with your iMac. First insert the Install DVD in your system and then restart it while hold down the C key. Once the disc boots and you have selected the language, pull down the Utilities menu at the top of the screen and select Disk Utility. This article will provide you with additional information about using Disk Utility to verify your disk. If you encounter unrecoverable errors you may need to seek help from a service provider.