Black screen on boot and loud fan, external display works, very slow

My iMac 27” late 2013 has a black screen on boot up, a very loud fan. I plugged in a second display works to show the desktop. But the computer is VERY slow and on investigation I see that the mac kernel is running the CPU at 250% or higher. The machine is so slow it’s almost impossible to use in this state. How do I restore the main screen and normal performance?

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Let's get a bit more info here. What happened? Did you smell something burnt? Did you open the system? Do you use a surge protector or UPS?

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