Hi Michele,
There are two places where single.php could be edited. One, in the core plugin version (which you should not be editing) in /wp-content/plugins/the-events-calendar/views/single.php and the other in an ‘events’ folder in your active theme. If the changes you made are not in either of those files, you might try looking in your theme’s functions.php file because you may have added it dynamically. Another thing I would suggest is getting a code editor that let’s you search across your entire theme folder or beyond and search for something unique to that enquiry form (a div or id or something like that).
I hope that helps!
– Jonah