Karlie here from the Shopify Guru Team! When I view your website on my desktop computer in my Google Chrome browser, I'm not seeing any issues with your storefront or dropdown menus.
Could it be that you're referring to the hamburger menu? A hamburger menu happens when the browser window is minimized or when you view dropdown menus on a mobile device. When the browser window is stretched out again to full size, the hamburger menu goes away and the dropdown menus work as normal.
You can see the hamburger menu in action in my video, here, where I minimize and maximize the browser window to show how the hamburger menu collapses your navigation menu. This is completely expected behaviour and is normal for a Shopify theme.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!