This is usually caused when an aquarium is not level or when the assembly is inferior. If you do not notice it in time, the whole panel goes plop on the floor and the result is most regrettable. More often, you will notice the problem before it becomes this extreme, although the floor will probably still be damp. Most seams will start to leak a little bit, giving you a chance to spot things before the entire contents of the aquarium is on your floor. If you need a new panel and you are working with a large tank, you may have a problem.
Thick glass is hard to cut.
Your local pet store might be able to get you a new panel from their tank supplier. However, many manufacturers will insist on doing the repair themselves. As with previous repairs, drain the tank, taking out all the equipment and fish first, and clean the tank. Remove the trim. This time you will need to remove all of it.