This is a problem you can fix yourself, every couple of weeks or so you should drain water from the cooler this can be done by removing the (usually it is red) cap from the overflow, which can be found under the pipe connections on the front of the cooler.
What has now happened is the water level has reached the point where it is running over the top of the tank.
If you take the cap off however and no water comes out. This means that ice has formed on the other side of the overflow, to remedy this; the cooler should be switched of whilst in use, until water does flow.
Then the cooler can then again be switched back on. Coolers should be drained or topped up regularly otherwise an ice bank will not form properly.