Really late to the party but I'd like to point out somethings.
To reset the parameter ram on a Mac you hold down cmd+opt+p+r and not control it has been this four finger salute since the compact Macs.
White screen can be a number of things so before you tear down your iMac which is a real PITA to do you should determine if it is something simple. Did the pram reset work?
If not and you have an external monitor handy and the Apple external monitor adaptor try plugging it in and see if you now have a desktop. If yes it might be an idea to backup or copy off any files and data that you might want should things go bad further on down the line.
If you did not get a working display on the external monitor you could try running an install DVD if you iMac came with one or if you can get a hold of a working iMac of the same generation you could try building a USB boot key by cloning the known good OS onto the USB key and booting your Mac with it. I recommend using Carbon Copy Cloner available here, https://bombich.com for this task if you build USB key. Typically though if the HD is bad or the OS is corrupted you will get an error on screen.
If you do see your desktop on the external monitor, go to System Preferences -> Displays and check to make sure that your primary monitor is selected. This is done by dragging the bar from the external display (2) to the primary display (1). Did this suddenly solve the problem?
If not it may be something a bit more involved. You could try reinstalling your operating system though it's also possible that your Mac has succumbed to a known failure issue which is a bad video card. Some folks have reported success with reheating the pins on the failed card using a mini butane torch. I have used this method on some Macs and PCs over the years and though it does initially work there is the chance that at some point the card will fail again. However if your iMac falls into this category and Apple will not help you out with their hidden extended warranties then this might be your only option if you feel adventuresome. Btw there are some who put boards in toaster ovens or full size ovens, personally I would not do this and if you went to a proper repair shop their will have an infrared rework station that is safer than using the micro torch method but also a more expensive option.
At this point though you are talking about getting really down and dirty with the insides of your iMac and this might be exceeding your comfort level. So if you think your iMac is scrap you might want to consider removing the drive and recovering what you can if the external monitor trick did not work. There are plenty of inexpensive external SATA hard drive cases or even SATA docks on the market that you can plug your hard drive into which can then be opened on another working Mac. Also a number of Youtube videos on the subject, you will need a set of small Torx screwdrivers as Apple tends to use these and Phillips on their computers.
I currently have an iMac A1224 on the bench with white screen, replaced the ram, reset PRAM as well as did a full reset (shift+command+p+r) and hooked up an external monitor. The external monitor shows the desktop fine and the Mac functions in this manner however the built in screen is still white which makes me think in this instance that the screen itself has failed.
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