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iMac 24 /2.8/2x1G/500/SD/WM/FRA , P/N: Z0E4002PN; Model# A1225; EMC:221
Problem: Screen start to blanking, Try to restart it by pressing power switch. Yes, it power up but hanging at initial screen and then when the processing bar reach 3/4 ; it restart by itself over and over again.
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I wrote a longer, more detailed reply but iFixIt's stupid auto moderation system flagged it as spam. Anyway, your hard drive is likely either corrupted or damaged. Turn the computer off, turn it back on and immediately press and hold the D key to boot Apple Hardware Test. Run the diagnostics to test the hardware. If the hard drive fails replace it. If it doesn't find a hard drive replace it.
Next, shut down the computer again and start it up. This time, press and hold Command and R to boot recovery mode. Google Apple Recovery Mode for info on how to re-install OS X. Even if you have a TimeMachine backup of your Mac don't restore that now - do a full new reinstall, then use Migration Assistant to restore from Time Machine.
If Recovery Mode doesn't work either the hard drive is corrupted or damaged. Get access to another Mac and use the App Store to download OS X. Google "create OS X boot USB" for info on how to create a USB Boot Disk from that downloaded file that you can then use to re-install OS X on your iMac.
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Thanks a lot . Trying now..
Tried Recommend steps #1: hold "D" key when power up, #2: hold command & "R" key when power up all no joy. #3: created a OSX boot USB then hold "option" key when power up the screen shows original HD, recovery HD, OSX El Capitan, & time machine drive. Tried to boot up by selecting original HD and recovery HD one by one all same. Tried boot from OSX boot USB the processing bar stuck at 2/3 then auto shutdown by itself. tricky..
Press "option" key when power up , on screen shows mentioned 4ea Drive. But slelect either one of them all unable to boot up to Login screen. Next step would be reseat hard drive to see how it goes.
Check your USB thumb drive on another system to make sure it's working correctly (holding the option key when booting up). If it works on the other system, your system has a deeper problem (likely needs a new logic board).
Dan, thanks the tips. Tried the USB thumb drive on a Macbook and it works .
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