In a community, the horizontal menu will disappear when creating a new Wiki or Blog, if you have enough text that requires to scroll down the page.
NOTE: Once the Wiki or Blog page is saved, the horizontal menu is visible- working as designed
1. In a community change Layout to 3 Columns with top menu or 3 Columns with top menu and banner
2. Create a new wiki page, add enough text that requires you to scroll down the page. (Do not save) and you will notice the horizontal menu will disappear
3. However, after you save and close the wiki or blog, then scroll to bottom of page, the menu is visible
HCL Development deemed this as working as design