iMac will not boot up

A few days ago whilst watching a movie on iTunes my iMac it froze

After rebooting it firstly became incredibly slow and although things like email were coming through iTunes and App Store would not launch. I also received error messages such as 'Preference could not load: Disk preference/internet accounts'

I tried a few recommendations including cmd + R but the first aid said fail and now after my most recent reboot the iMac starts up and shows the grey screen progress bar (as if an install is being made) but then shuts down

I have tried this now three times and it simply shuts down now