Mac won’t boot up

I thought I had taken care of it but no! Hubs told me it updated the IOS on its own yesterday, then it wouldn’t boot up. I can’t even take the update off because it won’t start. I started with the diagnostic key, it said there were no issues. Yea, but it won’t boot! I’d say it’s a serious issue! If I had bought AppleCare they’d refuse to fix it anyway. It’s 5 years old.