I was downloading different templates and trying them out and now have noticed that the default Protostar template is missing from the list of templates in the admin app. How do I get it back?
You can copy the template folder from the Joomla 3.4 full package.
Simply download the file, unzip everything (or only the
templates\protostar\ folder) to your Joomla folder. Then use the discover function (Extensions -> Extension Manager -> Discover) to discover and install the Protostar template again.