Vaclav:
Please show how you can accomplish what the OP wanted (!) , not what most replies wanted, in ONE line of code as can be done using WDT.
No, I won't.
Just because a solution is short, does not mean it is good.
I would use the non-blocking technique demonstrated in blink without delay to determine whether a game had timed out, and I would implement the game as a state machine that has its state reset when the timeout expires. This is more than a single line of code change, but that's how I would do it. Having to hard reset the Arduino to end a game suggests to me that the sketch architecture is wrong.