I owned a smaller version also known as an H 90. It's a 9'+ sailing dinghy made from the same material. When not in use I stored it up side down on wooden blocks. In its stored position it never had more than a foot of snow on it, but one spring I, too, discovered severe cracking. In other words, it doesn't take improper storage for cracking to occur. All it takes is a fast drop in temperature.
The primary crack was nearly straight down the middle of the hull starting at the aft end of the daggerboard trunk and continuing to, and then up, the transom. There were also various smaller cracks that branched off from the primary crack.
I found online a paper, prepared by Hunter, describing how to mend the cracks using Devcon's "Plastic Welder". This is a two-part "plexus" unlike epoxy, obtainable at any Walmart. My first attempt to repair the cracks was not a complete success. The areas where the bond failed required that I grind out the hardened plexus and clean vigorously with acetone and a stiff brush. The second time around I bought several more tubes to repair the smaller area than I had originally obtained for the whole project and, fortunately, such liberal use did the trick.
I bet it will work for you, too. It may not be fun, but you'll have a functional boat again. Once done, I suggest you immediately sell it to someone who lives in the South!