The valve body is a part of your transmission. It regulates fluid pressure to actuate the clutches that make your car shift. Some people power-flush their transmissions at the dealer, but I've seen this type of service destroy transmissions before. It's better to just change your ATF every 30,000 miles and your fluid won't get enough crap in it to get stuck in the valve body.

You may have developed a leak. Bad axle seals or rotted transmission cooler lines are usually the cause. Check your fluid level because low ATF will do the same thing as what you described.