Fix Non-Responsive Hamburger Menu on HTML Site

My HTML website has a critical issue: the hamburger menu does not open at all on mobile devices. This problem has arisen without any recent changes to the site, indicating a potential inherent issue that needs to be addressed.

Ideal Skills and Experience:
- Proficient in HTML, CSS and JavaScript
- Experience with responsive web design
- Familiarity with debugging mobile web issues
- Ability to conduct thorough website testing