Transmission woes. Why does my 5th gear come out?

It is due to the failure of the gear position locking mechanism. This is called a gear slip. It is not a big issue. It can be fixed by any local mechanic. He has to just have a look at the place where the gear stick gets connected to the transmission system. Since all other gears(1-4) are locking in position and working fine, it will not be a major problem.