You may be able to start in macOS Recovery to use the macOS Utilities options
there; even First Aid in the Disk Utility may help. However it also may be that an
issue in the startup process may need attending to directly... You could try Safe
mode startup, to see if that could work. It will also try to repair stuff then, too.
This may be a workable solution if system is there but has trouble starting.
• Use safe mode to isolate issues with your Mac - Apple Support
7 Mac Startup Options Every OS X User Should Know
7 Mac Startup Options ~ Every OS X User Should Know. ... The 'Mac Startup
Manager' works great if you have a number of boot options from which to choose, ...
https://www.tekrevue.com/tip/mac-startup-options/
Or try to start in recovery & use its 'Disk Utility - First Aid' to attempt repairs.
•About macOS Recovery - Apple Support
Note helpful 'disk utility' reference info here:
• How to reinstall macOS - Apple Support
{Choose within Disk Utility, to run First Aid, etc.}
[..Following older reference has nothing about using macOS Recovery version Disk Utility.
Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support
And relies on the user's vintage Mac and system install-restore DVD media discs..]
In any event...
Good luck & happy trails! 🙂