So, I'm super new to smartphones... but, I have been using the HD Razr Maxx HD for a couple of months.
That said, the device has completely locked up on me about 4 times. I have learned that you can force the device to reset by holding down the Power Button and the Volume Down button simultaneously for about 10-15 seconds.
Once you do this, you should be able to power the device back on, and everything should be back to normal. In any case, this worked just fine for me in these situations.
Not even sure why I am writing this, but I guess I was bored.