Volume slider goes missing and comes back sometime

I have an iPod touch 2nd Gen 32 GB. It was working fine for the last one year until a week back the volume slider disappeared while watching a video. The same thing is happening if I start listening to music. If I plug in my ear phones the slider comes back for a moment and then it disappears again. I tried to reboot it but it didn't work.

Can someone tell me whats happening?