You do not know which template or code you have accidentally messed up. That's why it is always better and safer to use a child theme to customize or modify your site.
Remove Horizontal Scroll Bar Bottom Of Computer Screen
Anyway, this is how you fix the problem of horizontal scroll-bar at the bottom of your screen.
1. Go to WordPress admin page.
2. Click at footer.php
3. Add this code:
<body style="overflow-x: hidden;">
just BEFORE the closing body tag </body>
4. It looks like this (as highlighted in red color):
<body style="overflow-x: hidden;">
</body>
</html>
5. That's all you need to do.