Virtually new iMac gets stuck turning on

[Glenavon Kate]

I have a 4 day old iMac which was professionally installed and has worked fine for 4 days. Today I agreed to a software update and when it tried to restart it got stuck. Kept cycling between blank screen and half blank screen. I turned power off and disconnected external hard drives and started up again. The apple sign comes up and that little sound and a noise from the drive. But it goes into a half black half grey screen then back to the apple sign and on and on. Tried three times.
Please help!!!
I had just bought Aperture online and spent 5 hours renaming my 800 iPhoto events with a date coding.

 

[simsaladimbamba]

Have you tried any of these steps?

Mac doesn't boot

Resolve startup issues and perform disk maintenance with Disk Utility and fsck

Troubleshooting: My computer won't turn on

A flashing question mark appears when you start your Mac​