gear shift

I was riding home tonight, and when I went to shift into third gear it wouldn't. I stopped retried and it still would not shift. Downshifting is no problem. I then looked at it, and realized the shifter isn't coming back up, so the rest of the way home I would have to lightly push down the front of the shifter in order to shift into higher gears. Does anyone have any clue to what is happening, and how do I fix it?

Thanks!