Ash here from the Shopify team. It’s great to hear that you were able to identify and resolve the issue!
Moving forward, I have some tips which can help prevent these types of stressful situations in the future. Whenever making code customizations on a theme, I suggest making a duplicate of your theme to use as a backup. Here is a link to our Help Center with some information on duplicating themes. When you duplicate a theme, the duplicate will appear in your list of themes in Online Store> Themes as “copy of (theme name)”. You can then publish that theme, or use it as a point of reference to see what the code looked like prior to customizations.
Another handy tip is knowing how to roll back sections of code that have had changes made to them. In this help document here, you can find information on editing your theme code, as well as reverting sections back to older versions. When reverting sections of a themes code, a drop-down menu shows the date and time for each save that you've previously made, including the original version.
Let me know if you have any questions about the above processes, or would like any additional assistance or tips.