Did this happen after you installed one the
Item Shop Mod that I made? If you managed to? If so, read carefully and make sure that you read and did ALL of things, or else you'd have this problem. Be sure to add the lines to your theme's header.tpl, and then, as instructed just two paragraphs down from that, open up inc/tabs.php and delete the contents of the file (but not the file itself). Missing any of that would definitely cause this problem.