I have this problem also, ever since the June security update. The local Samsung Experience Center was unable to fix and suggested sending to the repair center. I decided to pass on this suggestion.
HOWEVER, this is a fairly common problem as documented on the internet (with no solutions as yet). You should be able to restart the device by removing the battery for 10 minutes and then reinstalling and starting. The problem will probably reoccur when you turn off or restart the device. There's an app called WakeLock that keeps the CPU running and prevents the freeze - but does drain the battery faster.
Good luck.