Startup disk won’t work

Follow the steps at > https://support.apple.com/en-us/HT210898

In short, startup in macOS Recovery, select Disk Utility, choose All Devices, repair the Startup Disk.

If there is nothing to repair, then the disk has failed and needs to be replaced.

You're welcome.

If you have a good Time Machine backup, then see if you can restore those files.

https://support.apple.com/guide/mac-help/restore-files-mh11422/mac

If a lot of files are missing, then maybe start over and migrate everything from Time Machine.

https://support.apple.com/guide/mac-help/recover-all-your-files-mh15638/11.0/mac/11.0