Transmission - Stick Shift pops out of 5th. gear

I got a Pontiac Sunfire -1996 Model. It is a stick shift. I drove my car on the freeway and the stick popped out of 5th. gear by itself while driving. I was not attempting to downshift. So far I've not had a problem with my clutch or transmission. What's wrong? And what questions should I ask the mechanic and what procedure to find out the problem should I expect? Is this transmission fluid related? The service place want to charge me almost $ 500.00 just to look at the transmission to the determine the problem???? Please advise?

Siw ;-)