Hard to tell without any details. The least we need to know is your source material, driver versions in use, your project settings, what you were doing when the hang occurred, what you have tried to remedy the situation, what the events log tells you, what processes are running, etc. In short the whole enchilada.
Those messages normally mean your GPU is getting to hot and shutting down or going into a sleep statet o cool down. The Nvidia auto fan control is not aggressive enough to handle the heat from the MPE engine in longer projects especially on encoding. This is why we recommend manually setting the fan control to ramp in PP and AME at the least.
Thanks you guys ... my fans can definitely be cranked up. Hopefully that's all it is as my troubleshooting may have gotten myself into more issues, which I need to post shortly.