My MacBook Air is frozen trying to update to High Sierra. Please help

I have no cursor. the screen is black, flashing a white folder shape with a black question mark in it. I have a this year's 13 inch MacBook Air.

I was about 4 mins into updating to Os High Sierra.

simple turning off and on agpain has no effect.

I've put in an operating system number but don't really know what I have. It's the one it came with in January