Hi.. need help with reinstalling mac system OS

We can help you best if you will please tell us a bit more about your Mac.

Which model year is the iMac and what macOS version are you running?

How to reinstall macOS - Apple Support:

https://support.apple.com/en-us/HT204904

Turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.

If the recovery startup above doesn't work, the hdd may have failed and the recovery partition cannot be used.

IF that's the case, then try one of these startups to access internet recovery :

• If you use Option-Command-R during startup, in most cases you're offered the latest macOS that is compatible with your Mac. Otherwise you're offered the macOS that came with your Mac, or the closest version still available.
• If you use Shift-Option-Command-R during startup, you're offered the macOS that came with your Mac, or the closest version still available.

If it will not startup in Recovery (Command +R), then it sounds like you may have also erased the Yosemite Recovery Partition.

If it has a SuperDrive slot on the right side, then you will need to use the original Disc set that came with that iMac or a Snow Leopard Upgrade DVD to reinstall the OS X.

Yea' those vertical SuperDrives were not very reliable.

Once in awhile a good cleaning with dry cleaning disc helped.

If you have another Mac, you can make a bootable installer on a USB flash drive.

see > https://diskmakerx.com/download/

Yes, it can work wirelessly without a cable.

But it works better if you can plug in an ethernet cable.

Are you finding no path to anything but the flashing question mark? No other startup screen?

Ugh, yes. 2007.

Good catch. 🤦🏽‍♂️