iMac keeps rebooting

Try some of the following solutions for if your Mac that does not start up.

If your Mac doesn't start up all the way - Apple Support

If non of those get it going, call your local AASP and schedule a service visit.

https://getsupport.apple.com/repair-locations?locale=en_US

It is likely a hardware failure however by chance do you have any of the following third party apps installled?

• Antivirus
• Cleaning
• Security
• VPN
• Maintenance