Z3 Compact won't turn on

Hi, hope someone can help:

I was using my Z3 compact and the volume went down by itself, it then turned its self off. When I went to turn it back on I get 3 buzzes and nothing. After a while it is totally unresponsive to any of the suggestions on here e.g. hold power and volume etc. If I plug it in to charge, I get the red light and I go through the start procedure and it flashes up with Sony, goes into the charge screen but then the 3 buzzes again. I did manage to get it woking once for about 5 minutes but the volume turned down again and I kept getting the volume slider across screen until it shut down again.

Thanks