My Samsung Galaxy S2 was working fine today then after making a call, the screen wouldn't turn on and stayed black. I took the battery out after trying the home button and the lock button. I left it out for 10 seconds and put it back in and tried turning it back on. After several attempts of taking the battery out, putting it back in and trying to turn it on, I resorted to trying download mode by pressing the volume button, lock button and home button for 30 seconds and nothing. It won't even charge up when I try to plug it into the computer. What can I do?