I have the same issue on my store:
i restored theme.js file but it’s still the same
Hello @sophia123 ,
Open your store in chrome browser, right click anywhere in page, Click on Inspect, Click on console, Click on errors (errors are highlighted in red)
Now here you see all the JS conflict and out these one prevents you mobile hamburger menu to perform its action.
Once it's fixed your mobile menu will start working.
And to fix it you have to manually edit the theme files and find out from where it comes.