I don't know if you are using an IDE like netbeans or eclipse ? Each of those has their own file structure for the project for creating the project. The problem might also be your web.xml.
When the servlet is deployed to Tomcat \ webapps\ the reverse.html goes directly under the project directory
Here is my web.xml, please note my servlet is named reverse2 instead of reverse. Your web.xml needs to know where the html file is and where the servlet is. Your html needs to be in the <welcome-file-list> xml.