battery dead? Left sitting too long, battery over two years old, new?

If the battery is only 2 years old it should not have died just sitting in a car . There must have been a reason for it dying . I have an old truck which sits for 6 months of each year. The battery in it is probably 6-7 years old and it still starts after its long winter hibernation . Try putting it on a battery charger for 24 hours . The reason the horn was going off is the car alarm doesn't like being boosted . It thinks someone is stealing the car . If you charge the battery and it charge goes away I would look for some sort of parasitic draw . Hope this helps

PS @fishie I live in Canada and we get extreme cold which can be really hard on a battery and I think i get ripped off if I don't get at least 5 years out of a battery .