My Memeory stick has stopped working?

My memeory stick has stopped working, when i plug it in it is recognised as removable disc, but when I click on this it says please insert disc. I have tried to use on a number of computers but to no avail. When i go into properties it shows that the disc is full but last week there was only 25% approx. used. Even though it says it is full' it says there is zero bytes used.