Mac stuck after trying to install updates, startup disk missing
The generic advice when a Mac can't start is to boot to recovery HD (internet, local or external as is most convenient for you) and run Disk Utility to check for connection, decide to make a backup of critical files if needed, then attempt a First Aid - repair pass.


https://support.apple.com/en-us/HT204156


Assuming the First Aid works, you should be able to boot again or reinstall the OS from recovery. If not, you need to seek hardware repair, data recovery or try an erase install.

The reinstall instructions below get you to Disk Utility so you can look there and proceed.


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