Without knowing more about the issue it’s hard to offer any suggestions.
What do you mean the tabs are not visible, are you referring to the tabbed content or the actual list of links that form the links for the tabs?
Can you link to a page with the problem? If not, would a screenshot be possible?
Thinking out loud, i’d guess at a plugin or the theme setting some CSS that is conflicting with the tab CSS. You can rule this out by testing one of the default WordPress themes and disabling plugin’s temporarily.
Edit: No further feedback from user, marking resolved
