If you purchased your iMac between October 2009 and July 2011 then you could be at risk of hard drive failure. If you do not have an up to date backup then you are at risk of loosing data.
Apple are offering a hard drive replacement program.
See: www.apple.com/support/imac-harddrive for information on getting your drive replaced.
If you would like to discuss backups solutions for Mac OS X please contact us here.