My iMac went blank and will not turn on again.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

That being said see if you can boot into Safe Mode according to How to use safe mode on your Mac. If you can then uninstall Norton according to the developer's instructions. Then reboot normally.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

If that fails to work Boot into the Recovery volume (boot with the Command + R keys held down  - Intel Macs) or  (How to Boot an M1 Mac into Recovery Mode) and reinstall the system.

Get rid of any antivirus apps A.S.A.P. because they are the ones that typically cause panics and other serious issues.

You will find that without that extra, your Mac will run smoother than it has ever been.

As a result, I recommend service.

Service Checklist:

• Mac service pricing.
• Check AppleCare Coverage.
• Get your Mac ready for service.

Click ▶︎ Start a Service Request. ⚛️

Need more help?

• Call Apple Support at 1-800-APLCARE. 📞
• International Support. 🌐
• Contact Apple Support. 📝
• Chat using Messages. 💬

Click ▶︎ Check Repair Status. 🔧

Antivirus apps mess with the MacOS and aren't needed so it sounds that Norton has messed up your system.

The best option is to reinstall MacOS but to not migrate everything over from your Time Machine (if you have one).

Reinstall your essential apps but not antivirus or cleanup apps.

If you have your photos etc on iCloud they will be available on your restored MacOS

If you have other data files on external media they can be copied back if required.

How to reinstall macOS - Apple Support

Jblneonurse wrote:

My I Mac was on doing a clean with Norton it turned off and will not start again

I had that happen once with Norton. Took me the better part of a day to uninstall Norton and get my computer running.