macOS high sierra update installation

Hello,

I have just started installing the new mac os high sierra update, all was fine until the final installation. The mac restarted and started the installation, it gets to calculating the installing time which gets to 37 minutes remaining and every time an error occurs that prevents me from going any further. I had a power cut while it was installing which is what i fear may be one of the reasons.

The screen says there are options that can help to exit, but I can't click on it. I have tried the typical restarting ,but same thing happens again. completely turning the mac on and off the installing happens instantly so I have no way of accessing my mac or deleting /canceling the installation. Im not worried if the update actually installls i just need to be able to use my mac. Typically i havent backed anything up either.

Any suggestions will be hugely appreciated.