Must be the age of the battery. This has happened to me too. I had stored the boat at a boat storage service as I have been doing for a few years now, and when I took it out after winter I found that the battery is dead. The age of the battery was the reason for this. Disconnecting the battery when you put it in storage is also a good step. There are many such things for de-winterizing the boat, this article will tell you more about it http://www.migsonstorage.com/blog/jumping-back-in-the-water-how-to-de-winterize-your-boat/