I heard a funny sound just now and, as I approached my phone, I noticed that it’s vibrating. I thought that I’m getting a call but when I hit the home button to switch on the screen, nothing happened. The screen stayed black and it kept on vibrating. I tried holding down the lock/power key for a few seconds to turn it off completely, still nothing. I plugged it into the computer, hoping that iTunes will reset it, but iTunes didn’t even recognize that it’s plugged in.
Eventually I held down both the lock/power key and the home key for a few seconds and then the vibrating stopped. After I turned the phone on again, it seems to work fine. For now at least…
It seems that the problem was that I got a phone call 15 minutes ago. My phone is set to silent mode and to vibrate. Even after the caller hung up, the phone kept on vibrating.
So long story short: if your iPhone is continuously vibrating, the screen is black and you can’t turn it on, hold down the lock/power key simoultaneously with the home key for a few seconds. Turn it back on and that’s it. Hopefully it works for you as well.