Just a quick follow up…. it’s odd the Preserved HTML editor markup plugin caused that. I’m not sure if you found the plugin details about what I am about to say here….but…. it’s a tricky plugin to setup at first, so you need to read the documentation carefully for that plugin. Even I missed this once, but when you activate the plugin, go to Settings >> Writing >> select posts and pages to use
and save the settings, but click the tab buttons “Fix Posts” and “Fix Pages”. So now your Enter key will do paragraphs.
Before you do this, as it states in Red on those settings, do a site backup. You should check out WordPress backup plugins or use a service VaultPress. The most important thing to have in any website, is the function of doing full site backups on a regular basis. Backups are a lifesaver when you work hard and creating your site….and trust me, things do happen unexpectedly whether is’s something you did, or perhaps the web host server crash and burned, or you updated a plugin and it crashed your site….etc.