Craftsman Riding Mower Won't Start?

I have a Craftsman ride on mower 420cc. This was the first time I was going to use it this season. I go to turn the key, nothing. So I hook up a battery charger to it and let it charge 4 hours. I go to start it. It sounds like it's going to turn over, then nothing. Then I just hear a clicking sound. Any ideas? Maybe the battery is no good? It is a 3 year old battery.