How to turn on iMac?

Question

macOS

On a computer one axis number ten with something. The third time I try to start, the loading indicator fills up completely and gets stuck on this: nothing has happened for more than 20 minutes.
The previous 2 attempts tried to reboot, as you can see, it does not help.

Rebooted again, in the process of turning on I pressed cmd + v:

Answer 1

Found this way, how safe is it?

CMD+R at startup, select Utilities->Terminal from the menu
In the terminal, type 2 commands:
cd /Volumes/Macintosh\ HD/var/db/caches/opendirectory
rm mbr_cache
Reboot,
It takes longer to load the first time, but it loads!
Then it loads as usual.