I have witnessed a similar lock up last night on my radio. First off, I'm running a NX-5700/NX-5800 into a single, full featured control head. My radio was on the VHF band, which is set as my primary band. I was in scan mode and I manually changed channels to a DMR simplex channel to communicate with another vehicle (car to car). While conversating with the other vehicle, about 3 transmissions in, I was unable to re-key my radio, I could not receive anything nor could i do any other functions on the radio. I was forced to shut down the radio manually, via the power button and restart the radio. Once I did that, the radio seemed to function properly again.
I am running the latest software and firmware.