Why does my website sometimes work and sometimes not?

Hi Everyone, New to shopify, but some help would be widely appreciated.

Simply, sometimes my site works, some times it displays a message saying I need to upload my website to my ftp - I’m fairly certain that that it’s something to do with the secured server.

some people don’t see the site at all, some people do, I can click the same lnk twice and have the same result.

I really appreciate any help.

Craig