To reset the PRAM (parameter ram), hold down the Option, Command, P, and R keys while powering on the iMac.  You can release them after the first chime.  You'll likely notice this will change screen resolution to its default setting once the desktop comes up.

To reset the PMU (power management unit), you'll need to turn the computer over to access a special button located under the RAM access panel.  This linked image shows and describes how:  http://mrjcd.com/junk/PMU.jpg

Hope this helps!