Touchscreen MP4 hangs when a video selected
 

My wife's touchscreen MP4 player was working fantastically and then all of a sudden has started to hang when I try to open a video file.It used to play videos brilliantly but now is a dead duck.When it hangs it wont even switch off so I have to press the reset button on the side to enable a restart.It was playing music fine but even this is no good now.Please help I have tried the person who sold it but get no reply.It is only 4 months old.