Mac shows No Entry Sign after booting.

Bit of a weird one. Last night I left my iMac on with a few windows open as I went out. In the time I was out there was power cut. I didn't think much of it so left the computer off. An hour or two later it boots up by itself and then stops. It then boots up again. In the middle of this boot up I hold down the power button to turn the computer off.

I thought nothing more about it and this morning my dad went on the Mac to go about his work. I went back onto it this morning and logged off his user and logged into mine. As o was in the middle of something last night all the old windows tried opening. Google chrome wasn't starting and something about icloud popped up. I then closed the windows and restarted the computer to give it a clean boot so to speak.

When rebooting the loading bar went most of the way before going black and shutting down. I then tried it again with the no entry sign appearing then.

I've been into the disk utility and seen that my main macintosh HD is not mounted nor will it allow any first aid. I also tried to update or reinstall the operating system on the same panel and it would not because it couldn't reach the server. There is also an is x base system disk image which I'm not sure I've seen before or know exactly what it does.

I'm at a loss as to what I should do now?

I apologise for such a long explanation, but I hope me recounting everything that happened might help. Thanks in advance for your help.