A headless Mac Mini server gets stuck (non-SSH-able) after rebooting (macOS Mojave)
The obvious thing to check is if you have FileVault enabled and attach a display simulator dongle. In the FileVault case, the machine does not restart to the OS but to a firmware / limited log in screen, but having a display connected won’t enable/disable FileVault.

You will want to have a remote KVM or someone that can log in for unexpected power failures. See this thread for a lot more options like power, display and management options for headless Macs.


Remote into Mac Mini after a reboot
Rebooting a headless Mac Mini