website being hacked

we used our website for a golf club…it archives scores ,pictures history of the club …rules of the club …our site also has a forum tha club members can post stuff on…to get into the forum there was supposed to be approval by me the admin we have only 32 users…at one i noticed 450 users …also saved pages on the home page of the site have been deleted and me the admin did not do it …i need help to combat this prob lem and fix it