deleted code and also lay-out

What theme are you using? Where did you download it from?

twentytwelve from wordpress

Re-upload a fresh, unpacked, copy of the theme’s folder to wp-content/themes using FTP or whatever file management application your host provides.
https://www.ads-software.com/themes/twentytwelve

In future, do not edit the theme itself. First create a child theme for your changes. Or install a custom CSS plugin.

OK, will I lose all of the changes that I’ve made?

Yes.

Is there an alternative? I’m sure there’s something I can replace those two lines of code with that should work? Its a bit drastic to lose two weeks of work over. I’ll definitely install a child theme for future, but I’d like to not lose my changes in the meantime.

You are just missing a second bracket at the end of your code –

#nav > div > ul > li {
background: url("https://studyathomemama.ca/wp-content/uploads/2014/07/yellow-flower.png") no-repeat scroll right 14px transparent;
padding-right: 20px;
} // - add this, n everything should recover