My site is all wrong

I went to look at my site and all the colors are inverted and our header isn’t showing up.

I also tried to log in through wp admin but it says “can’t access file.”

I don’t know much about web design or coding, and this is my company’s site, and I am required to fix this.

Any idea what’s going on here? I suspect a faulty plugin, but I can’t even log in to fix this.