Upgrade to Catalina

Michael1109 wrote:

I tried to up grade from Mojave; all was going well then I got the circle with a slash when I tried to reboot. How do I get out of this dead end?

Boot into Recovery (Command R) and from the dropdown menu: Utilities>  Disk Utility> run the First Aid on your Macintosh HD (and the "Macintosh HD-Data" volume as well if Catalina) If errors are found and repaired, run again until no errors reported.

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

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904

Your only option at this point is to attempt to launch recovery mode (Command R) or internet recovery mode (Option Command R).

If you don't have any data you care about, or have it backed up, you'll want to erase your hard drive in disk utility (select View menu, Show All Devices in Disk Utility if you have that option, and aren't seeing your physical "APPLE SSD" or other physical drive at the top of the list, and select Erase.

If you do have data you care about and that's not backed up you'll need to pursue data recovery options unfortunately and will need to switch gears from trying to fix it. (taking it to a technician, or using Target Disk Mode with a 2nd Mac and data recovery software or simply attempting to read the files from the other Mac are the two main recovery options)

If you do have data you care about and that's not backed up, but can't pursue any data recovery options you can attempt to not erase it at all, just run a First Aid scan if you can still see the drive (you may not be able to) in Disk Utility, and attempt an over-top reinstall. This is very unlikely to work so it would be a very last ditch effort if you have no proper options for a technician or self-data recovery with Target Disk Mode.

What model Mac are you doing this on?