Help with gearbox

Last night I was driving home and I thing the gearbox got broken while I was driving in the highway. When I say broken I mean that third gear was put because I was driving in third but it was not possible to put any other gear, as the gearshift was 'dancing' freely in the gearbox. Has this happened to anybody else and what should I aspect from the mechanics. I've had the car since new, the only owner and it only has 49,000 miles, it’s a 2001 325CI with manual transmission. Any help will be appreciated.