I understand that when you start up your Mac, you see a circle with a line through it. I'm happy to provide some help for this issue.
I recommend going through the steps provided for this in the article below:
About the screens you see when your Mac starts up - Apple Support
When you see a circle with a slash symbol instead of the Apple logo, it means your Mac couldn't find a valid System Folder to start up from.
If you're using your Mac at a school or business, it might be trying to start from the wrong version of macOS. Contact your IT department for more help.
If this is your personal Mac, try reinstalling macOS using macOS recovery.
Thanks for using the Apple Support Communities. Cheers!