I have a circle with a line on my iMac.

If your Mac starts up to a circle with a line through it

A circle with a line through it means that your startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. 

A prohibitory symbol, which looks like a circle with a line or slash through it, means that your startup disk contains a Mac operating system, but it's not a version or build of macOS that your Mac can use. 

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Use Disk Utility to repair your startup disk. 
3. If Disk Utility found no errors or repaired all errors, reinstall macOS.
4. If you still need help, please contact Apple Support.

This means that your Mac is unable to find and open its operating system. What happened prior to this occurring? The operating system will have to be reinstalled. When downloading, do not use Wi-Fi as it is too slow and almost always fails. Use an Ethernet connection.