404 Error on all except home page

I’ve just gone to my website to find a link to share on a social media post.

except I got a 404 error. I checked the home page – it works fine. but Clicking on any other page/link returns a 404 error.

could anyone suggest how I fix this?

Unsure if its related – I have been receiving Error 500’s infrequently over the past two months or so.

The page I need help with: [log in to see the link]