Why won't my phone switch on?

My Samsung Galaxy S II gt-i9100 phone switched off by itself.I had bought it second hand.The previous owner had rooted it.It was running a nightly build Cyanogenmod OS(ICS Android). I put it on charge for more than an hour but didn't switch on even then.Tried different batteries.Didn't work.Tried to start it in recovery mode by pressing volume down+power+home buttons.Nothing happened.It has not taken any impact and never been dropped in water.I can't tell if this is a hardware problem or a software one. cyanogenmod had been prompting me for a long time to update the OS but i didn't.