imac not loading

You could try Safe Mode, and an NVRAM and SMC reset, but you’re probably headed for a hardware repair. If you do manage to get this Mac booted, your absolute first priority is to get a complete and current backup. If this is a hardware failure, such as a failing hard disk drive, repairs and diagnostics can mean less data recovered as the storage dontinues to,degrade.

Again, I’d suspect hardware failure.

Assuming the NVRAM and SMC reset did not alter behavior, and assuming that Safe Mode did not work, and assuming there are no external devices connected to whichever Mac this might be—other than keyboard, mouse, and/or monitor, if you’re using those...

...then you could try booting Recovery,

...and maybe then using another Mac to build an external bootable installer and test-booting that external storage device.

But this pattern usually means this Mac is headed for repair or maybe replacement.