iMac boots with the line 1/2 accross then after a minute circle ⭕️ with a line through it command R gives same results

What year model iMac?

What macOS is it running?

Follow the steps at > https://support.apple.com/en-us/HT210901

If you are using a wireless keyboard and can not get it to boot into macOS Recovery Mode, then try using a USB keyboard.

Likely failed hard, read the article Jack's link offers. Then after testing, please take the computer to your local Apple Authorized Service Provider to be professionally diagnosed and then you can decide. Apple Authorized Service Provider Locator (AASP) & Genius Appointment

Hi Ldolliso,

Please read: If your Mac starts up to a circle with a line through it - Apple Support.

Cheers,

Jack

I've had the same problem on a 2017 iMac Pro. It has never happened until the latest Mojave security update (2021-003) was installed. I'm sure that broke something. At first I couldn't shut the machine down. Now I get the prohibitory "circle with a slash" about fifty percent of the time when starting up or restarting. Did everything in the support article (Recovery, Disk Utility, re-load the OS). Also reset the SMC and NVRAM. Nothing helps. I believe that it's more than coincidence that this all started after installing the Mojave security update.