What Causes a Flight to Vanish From the Air Surveillance?

The flight took off, I think late at night. It was supposed to go all the way in the north to Beijing. The transponder, which is an instrument of the plane, was switched off. So it fell off the radar and then there's a mystery as to where it went. There are two arcs that are possible. One takes its southwards into the middle of the Indian Ocean,. Where obviously there's no place to land - meaning it would have crashed there. Another possibility is that there's another arc taking it all the way into Kazakhstan.