My NW A35 has become unresponsive  I charged it through my pc and ten minutes later when I came to use it, it had seemingly switched off as the buttons do nothing and the screen is black.

I've charged it (the red light still shows) but it still won't turn on. 

Any ideas?

Edit: it's suddenly sprung into life so the immediate problem is solved but I wouldn't mind any thoughts on how I could fix it if this reoccurs.