One of my AFPS drives has disappeared.

Hi I rebooted my IMAC that was running VENTURA and it came up in my MOJAVE disk and the message 'Incompatible Disk This disk uses features that are not supported on this version of macOS'

I decided to go to SYSTEM PREFERENCES/BOOT DISK and select VENTURA, but it isn't shown as a boot option. If I go to FINDER/COMPUTER I have MACINTOSH HD - DATA, MOJAVE & VENTURA. The HD - DATA has all my files that were attached to VENTURA and I also have a full backup on Time Machine. What has gone wrong and what should i do?