I've heard of this happening from time to time. Popping the battery out seems to solve the problem every time. I guess that DSLRs are complicated computers and, just like your computer at home can occasionally freeze, the same thing happens and a hard reboot is necessary to reset it !
: 15 Ltd, 77 Ltd, 43/1.9 Ltd, Cosina 55/1.2, DA*300/4, Contax Zeiss Distagon 28/2.8, Raynox 150/250, AFA x1.7, Metz 50 af1.
: D800, D600, Sigma 500/4.5, Sigma 120-300/2.8, Zeiss Distagon ZF2 - 21/2.8, Zeiss Distagon ZF2 - 35/2.0, Nikkor 85/1.8G, Sigma 50/1.4. Nikon x1.4 TC, Sigma x2.0 TC