Hi Liza,
What Leo mentioned may have been the case. It it is indeed what happened, I’m afraid we can’t recover the previous files that were erased when the theme updated.
You’ll have to get the appropriate google tag manager code again from Google and place it abiding by good practice it won’t be a hassle in the future.
Reminder: In the future, please make sure to use a child theme’s functions.php or a Code Snippets plugin to add your codes in so this doesn’t happen again. 😀