Help me , my I Mac will not start up

Try starting your mac up in Safe Mode,

How to use safe mode on your Mac – Apple Support

Safe Mode takes a long time to start up so be patient.

It is running disk repairs and clearing caches.

If it starts up in Safe Mode then try restarting normally.

Things to try...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

 Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold ...

Command (⌘)-R

Installs the latest macOS that was installed on your Mac.