If you need a simple responsive CSS only navigation menu you will love this one by Mayur Suthar. On mobile, the menu turns into a hamburger menu and on clicking it the menu turns into a drop-down.
If you are having trouble with the pen, try the archived copy on GitHub
Responsive Navigation Menu Snippet
See the Pen Responsive Navigation Menu Using Only CSS by Mayur Suthar (@mayursuthar) on CodePen.0