Updating Themes

Hi, I am using a customized version of the “Unite” theme. I kept seeing the message that I should update the theme to the newest version. So I did and now several things have changed. Text is not formatted correctly, all social network icons are gone and the footer is gone. I assume that since this theme was customized, I should not have upgraded my theme? Is that correct? I did not do the theme customizing, I paid someone to build the site.
I also see that a new version of WP is available and the message says I should upgrade now.
When does it make sense to upgrade themes and versions and when does it not?
Thank You,
Peter