My wii freezes in-game and won't respond at all

it all happened when i played tennis for over an hour and it suddenly stopped. It emmited a low pitched sound. Now it happens every 30 mins of play and does the same thing all the time there is no way of unfreezing it as the off button wont respond neither will any other of the buttons. The wii controller says it is still connected. The only way to switch it off is by unplugging it. If i switch it on a minute later it takes about 3-5 mins to freeze again but if i leave it longer than an hour it will only stop at round about 30 mins. help.