Are you on Win10? Because win10 has a common problem with freezing up randomly while playing games and you then have to power off and reboot the computer again. Seems to happen when you hit many inputs at the same time especially while playing local multiplayer.
This has become a real problem on my game and I can only hope that Microsofts solves the issue soon as people don't understand that it is windows10 fault and not the games fault.
Haven't found any working solution yet but there are tons of threads about this random freezing problem all over the internet and on many game forums on Steam.