Full progress bar but no login screen
I have a Mac Mini that is probably due replacement but has been running fine and is fully updated. Until this fault suddenly occurred a couple of months ago and I have been unable to access it since.
When I start up the computer takes a long time to complete the process bar, maybe twenty minutes, but when the progress bar is full it does not move on to the login screen. I have left it for hours, but with no progress.
I have disconnected external hard drives and only have connected a monitor via HDMI, a USB keyboard and USB trackball. No change.
I have tried restarting while holding down the shift key, for Safe Mode, but it still acts just the same and does not take me to the login screen.
Any thoughts?