During use my iMac showed yellow spots that later turned into 5 vertical stripes. I shut it off, waited, then tried to reboot—no success so I shut it off again and zapped the PRAM; but as it tries to open, the little bar below the Apple icon progresses from left to right about 2/3 of the way across and then the screen turns grey.
iMac, OS X El Capitan (10.11.4), 2.66 GHz Intel Core i5, 4 GB 1067 M
Try a Safe Boot by holding down the Shift key while starting up the Mac.
Thanks Rysz,
Unsure of how long to hold down the Shift key, I held it down till the little bar progressed to the 2/3 point where it usually went to a blank screen.
I continued to hold for more than a minute then released and the little bar verrrrry slowly moved all the way across and now I was able to reboot.
Oddly, the stripes remain and are medium blue where the screen image is dark, and yellow where it is light in color.
Sorry, should have explained that you only need to hold the Shift down until you hear the startup chime and yes, booting into Safe Mode is slow, as the OS is doing a bunch of cleanup work.
After the Safe Mode boot is completed, restart normally. You can also try it a second time.
This might be a hardware issue...
Thanks again!
I hope clicking "This helped me" shows up as crediting you fully—I really appreciate your help.
My iMac developed yellow spots and then stripes, froze and won't reboot
