If you CPT is called “work“, i.e. you have registered it using
register_post_type('work', $args);
you have to create a file called 'single-work.php' and that will be used to show the singular work post.
If you don’t create that file the file 'single.php',if present, will be used by WordPress. If even the 'single.php' does not exist in your theme, then the 'index.php' file will be used.
Please see WordPress Template Hierarchy on Codex for more info.