Try Booting on Safe mode
Before restarting your iMac on safe mode make sure you had only connected to necessary items like keyboard and mouse. Safe Mode may be a diagnostic mode designed to repair macOS problems. Thus Safe Mode is often a useful troubleshooting method.
How to boot iMac on Safe Mode
Shut down your iMac and wait a minimum of 10 seconds.
Restart your iMac. And immediately, press and hold the Shift key as your computer restarts. you would like to press the Shift key immediately after the startup sound/chime before the Apple logo appears.
Release the key once you see the grey Apple logo or the login screen. Then log in, please note that you simply may need to log during a few times.
Be patient because safe mode is slower. if you iMac starts up, restart your iMac normally without pressing any buttons. don’t press the Shift key when restarting.
Reset PRAM or NVRAM
You may want to reset “non-volatile random-access memory” or NVRAM (this is additionally called PRAM). Follow these steps:
Turn off your iMac.
Wait a minimum of 10 seconds.
Turn on your iMac.
Press and hold the Command-Option-P-R keys together immediately after the startup sound.
Your computer will restart (keep holding the keys)
Release the keys once you hear the second startup sound. This process will reset NVRAM.
Reset SMC
Resetting the System Management Controller (SMC) may fix your problem. Here is how:
Disconnect the power adapter.
Press the power button for more than 10 sec
Then release the power keys at the same time.
Connect the power supply
Reboot your computer
Recovery Mode
If all of the above’s options didn’t fix your iMac, then you should know the issue is complicated, Reviving the iMac through recovery mode may risk your data. Make sure you have all the backup. If you don’t contact us our technician will help you to fix your computer without losing data.
Repairing the Disk in Recovery mode
You can try repairing your startup disk by opening Disk Utility from macOS Recovery. We do this because it’s possible that the disk could also be corrupted. Please follow these steps:
Turn off your iMac.
Restart your iMac and hold option+R keys immediately after the Apple startup chime. Keep holding the buttons until you see the Apple logo.
When your iMac starts up, you’ll see the “macOS Utilities” window.
Select Disk Utility and click on Continue.
Select your startup disk (by default, it’s called Macintosh HD, unless you renamed it).
Click the primary Aid icon then click the Run button. Disk Utility will now attempt to find and repair any problems.
When you are done, then reboot your iMac.
Reinstall the Operating System
Reinstalling macOS may fix the issue but this may also delete all the data from your hard drive. You’ll use macOS Recovery to try to to this. Here is how:
Shut down your Mac. you’ll get to restart your Mac from macOS Recovery. There are a couple of options you’ll choose. Here are the startup options:
Restart your Mac and immediately, press and hold the Command-R keys until you see a spinning globe: this feature will reinstall the newest macOS that was on your Mac. We recommend this feature. this may reinstall the macOS version that’s stored on your Mac.
Restart your Mac and immediately, press and hold the Option-Command-R keys until a spinning globe appears: this may upgrade to the newest macOS.
Restart your Mac and immediately, press and hold the Shift-Option-Command-R keys until you see a spinning globe: this may reinstall the macOS that came together with your Mac when it had been bought. Select Reinstall macOS, then click Continue then follow the onscreen instructions.
If none of these troubleshooting options fixing your iMac, then you know maybe the hard drive, graphics card or the CPU is bad there. This doesn’t mean that the problems are not fixable, visit Mac.Infinity store to get it fix on the same day.