My iMac now opens to blank screen.

I ran disc repair on my imac 27”. I ran the reinstall system without erasing the HD. I had been running Ventura but the reinstall loaded the original Mohave OS. After that I updated to Ventura. Computer now opens to blank screen. When I restart with command r I can see that the Fusion Drive now has two parts. Macintosh HD-data and one named “update”. I’ve spoken to Apple tech twice and they have not given me a way to fix this. Obviously, I did something stupid. Anyone have answers on how to fix this? I don’t know what to do!

[Re-titled by Moderator]