Some more information required: What is your Pi model and the Volumio version?
Are you on a fresh Volumio system or did you already take some steps to get the MHS-3.5 working? From your post I can only assume that you probably have installed the Touch Display plugin and now the display is showing Volumio’s UI but touch is not working. Please describe in detail what you did to get the display working. If you should have experienced error messages: What did they say?