Which iMac do you have? Year and model? What version of OS X is it running? If you have an install disk, boot with it, select utilities from the top menu, select disk utility and verify the hard drive. If you don't have an install disk and are running Lion, Mountain Lion, or Mavericks, start your iMac by holding down the command and r keys. That will get it into recovery. Once in recovery, select utilities, disk utility, and verify the hard drive.
And... please do not cross post your question. Only post to one forum. Your duplicate post has been removed.