I had the same exact problem today! I called Technical Support and talked to a very helpful woman! She explained to me that sometimes your phone needs to "reboot" itself. To reboot the phone, I just held down the power/lock screen button, and the volume down button for about a minute. The phone then restarts and gives you the power up screen. Now my phone is back to normal, and I can unlock the phone without it needing to be plugged into the charger!