Restarted after freezing and now it does not turn on?

My Galaxy S2 got slow and freezed so that I had to turn it off and on again. I tried to restart the phone but it stuck after the "Samsung" Logo. However, I was able to enter the hard reset menue, where I wanted to do a "wipe data/factory reset". I can remember a "E: Can't mount /cache" message, however, I started the wiping process and after a few minutes the display turned off and nothing worked anymore. I tried two differend batteries, different cleaning procedures but nothing helped. How can I see what is broken (mainboard, display, storage...)?