For some users who have the latest iMac (mid-2011), startups have been rocky, including getting stuck at a white screen or dealing with the appearance of vertical lines across their screen. In this case, you must install the latest iMac firmware update,.
If you have not installed the iMac Graphic Firmware Update 2.0 and find that your iMac is unresponsive during startup, you should reinstall the firmware update. Remember that firmware updates are different than normal updates and require some interaction with your machine.
Download the iMac Graphic Firmware Update 2.0 and follow the instructions to load it successfully onto your machine. Remember to keep your computer turned on until the update is completely done installing, as turning your iMac off in the middle of the install could damage it.
Should you experience hangs while attempting to wake your iMac from Sleep Mode, installing the iMac Graphic Firmware Update 2.0 should help alleviate that issue as well.