It probably wasn’t the WP upgrade – it would be the theme upgrade which you probably also upgraded at the same time.
If it upgraded the theme, and you didn’t backup – you’re hooped. Some hosts will have backups (telus hosting)… other wise have fun coding again unfortunately.
Perhaps create a new stylesheet and call it so that it doesn’t get overridden next time you update your themes.
Modifications to the default WordPress theme will be overwritten every time the theme receives an update. It’s usually recommended that theme modifications be made in the form of a child theme, to avoid such issues.
It’s usually good practice to keep a local backup copy of any modified files for just such circumstances. It’s also good practice (and not just for WordPress) to take a complete backup of your sites files and database before performing an update/upgrade to avoid these unfortunate issues.
If you didn’t make any backup copies of your modifications, or of entire the site, I think that unless you can find a cached version of your actual style sheet somewhere, it might be gone. Is there any chance your host might have a backup of your files?
You might be lucky to find a a Google cache of the site? I think those last a few days. Might be able to copy the stylesheet similar to what ClaytonJames mentioned.
