First things first, double-check your internet connection is working.
Next, you need to make sure you've selected an answer before you can move on. With a slider, this means moving the toggle. For a dropdown question, make sure you've clicked on one of the options. If both those things look good, we'll need more info to help you out.
You should see a little speech bubble icon in the lower right-hand corner of your screen. Click on there and tell us what's happening including: Any error messages you're seeing, the name of the browser you're using (chrome, safari, internet explorer etc), and what kind of device you're on.
We may come back to you with a few more specific questions to help us re-create the problem and find out what's happening!