If your tabs are mysteriously disappearing after a recent upgrade to Sharepoint 2013, you might be wondering why. If so, you are in luck because it may be a simple fix.
I found my issue stemmed from web parts on the library/list app page. Sharepoint 2013 provides you with a way to add web parts to the app pages. However, when web parts are added, the ribbon options disappear because it’s no longer just a library page. Now it contains other web parts as well. My suggestion is to remove the web parts so that users can have their ribbon functionality back. Then place the needed web parts and the library/list web part onto it’s own page and use that as the landing space. That way users can still upload documents and use the list’s ribbon as intended!