My iMac does turn on but after a few seconds it will turn off and that’s it

You can attempt to restart in Safe Mode ( Use safe mode to isolate issues with your Mac ) and then restart normally. If that does not work, then you need to take it to your local Apple Authorized Service Provider to have it diagnosed and then you can decide what to do.

Troubleshooting Mac Startup or Power Up Problems

What to Do When Your Mac Won't Power Up At All

What you describe, if correct, is sometimes an indicator of a failing power supply or other hardware component like RAM or the disk drive.

Make an appointment at the Apple Genius Bar for service after the stores re-open. If you need to find an Apple Store - Find a Store - Apple. Also, see How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD). You may also get Apple authorized service at a local Best Buy.

Contacting Apple Customer and Support Service

1. Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
2. Primary support help at Contact Support.
3. To contact product and tech support:  Contacting Apple for support and service including international calling numbers.