screen goes black?

So our wii is not connected to the internet. Sometimes it'll say 'update' and 'update failed' I guess because it isn't connected? But sometimes it will work anyway. Well today when my sister put in a new game the screen just went black and she tried several times of holding the button down to turn it off, like on a computer. It worked after several tries, then we waited a minute and turned it back on and it worked fine. Anyone know why this happened and how we can prevent it from happening again?