Press and hold the volume down and power button for 15 seconds or until the device powers down. With the GDR2 update this problem is common when double tap to wake up the phone is enabled.
This has happened to my Lumia, and I do NOT have the double tap option enabled. I DO have the Glance feature active as I find that useful and convenient; the lock up has only happened to me once. Removing the SIM allowed the phone to restart and the touchscreen worked again. Reinserting the SIM and restarting got me back to a working phone, although the data network had switched to Global and I had to reset it to LTE/CDMA.
As far as what causes it - I really don't know. It seemed to happen randomly - took the phone out to check messages and it was frozen and unusable until I was able to restart it by removing the SIM.