Since the OP hasn't answered the question, I'll just put the answer out here for everyone else's bennefit.
The secret is: If you use the correct language to describe the 'bar', it's easier to figure out how to turn it on/off. ;)
In Word and other Office products, it's called the 'Ruler', not the 'Tab Bar'.
So, if you're using Office/Word 2003, you turn it on/off using the View menu. Click View, then click Ruler. It's an on/off switch. If it's checked (on), clicking it will turn it off. If it's unchecked (off), clicking it will turn it on. When the Ruler is turned off, it hides and can be made visible if you mouse over its edge.
In Office/Word 2007, which uses the 'Ribbon', the command might be located somewhere else. I don't have a copy of 2007 to look it up for you. But, you're still looking for the 'Ruler' to turn it on/off.