Too many iMac variants out these so we need more info. Please do "About this Mac" from your Apple menu and post what it says about your Processor type and Mac OS version. That will help narrow the field. When you find the OS version, please change your equipment profile-- "iOS6" cannot run on any Mac computer. Fleshing out your equipment field will get you help faster as we don't have to go the "probing question" thing to start analyzing your issue.
Note: this forum is for iMacs made before 2006. Through a bone-headed labeling decision on Apple's part, many owners of newer iMacs end up here by no fault of their own. If you know your iMac is newer than 2005, or "About this Mac" says you have an Intel processor, consider reposting in this forum: