Open a TAC ticket if you have not already: https://www.netgate.com/tac-support-request
Do you know what versions it was upgrading between?
It sounds like it stopping at the bootloader prompt though which implies it cannot see anything to boot from. We'd have to see the output before that to know exactly what errors are preventing it.
A clean install of 24.03 is going to be the fastest way back, especially because there is no config on it yet.