When my MAC start, normally appear the Apple Symbol and in the same time a bar under below in progress until the system remain ready. Sometime the bar don't appear and the procedure stop. I have to switch off the system and restart 1 o 2 times before to have the PC ready.
[Re-Titled by Moderator]
iMac 27", OS X 10.11
Boot into Recovery (Command R) and from the dropdown menu: Utilities> Disk Utility> run the First Aid on your Macintosh HD (and the "Macintosh HD-Data" volume as well if Catalina) If errors are found and repaired, run again until no errors reported.
Recovery: https://support.apple.com/en-us/HT204904
https://support.apple.com/guide/disk-utility/repair-a-storage-device-dskutl1040/mac
Boot into Recovery (Command R) and from the dropdown menu: Utilities> Disk Utility> run the First Aid on your Macintosh HD (and the "Macintosh HD-Data" volume as well if Catalina) If errors are found and repaired, run again until no errors reported.
Recovery: https://support.apple.com/en-us/HT204904
https://support.apple.com/guide/disk-utility/repair-a-storage-device-dskutl1040/mac
Sometime the loading bar doesn't appear and the boot up procedure stops
