Whole layout has disappeared

Hi,

I've been working on my site (not technical, writing articles only) from the backend over the weekend and everything looks fine from there. When i preview an article as i'm working on it, it everything is exactly as it should be.

I just logged out however and opened my website 'from the front end' and it's all a mess. From the front end all of my formatting is gone on pages and posts and the header is a disaster. I cant see this from the backend as everything is still perfect. I don't understand whats happening?

On mobile the content formatting looks ok but the header is still messed up.

Any thoughts on how to fix it?

Thanks,

Christine