iMac Manual
Page 81
...doesn't work, press the power (®) button and immediately hold down the Option key and press the power (®) button again to www.apple.com/guide. If the iMac won't turn on or start up , open System Preferences and click Startup Disk. Then hold down the Command (x), Option, P, and R ...you may need to use Disk Utility (located on the Mac OS X Install Disc 1 that came with Your iMac" on page 82). If the problem occurs frequently, you may need to start up (see a flashing question mark  Wait a few seconds. Check whether removing the memory you installed allows the...
...doesn't work, press the power (®) button and immediately hold down the Option key and press the power (®) button again to www.apple.com/guide. If the iMac won't turn on or start up , open System Preferences and click Startup Disk. Then hold down the Command (x), Option, P, and R ...you may need to use Disk Utility (located on the Mac OS X Install Disc 1 that came with Your iMac" on page 82). If the problem occurs frequently, you may need to start up (see a flashing question mark  Wait a few seconds. Check whether removing the memory you installed allows the...