It probably cracked because it flexes at that point, and will need significant reinforcing if it's not to occur again soon. Is there a corresponding crack or loose joint on the inside as well? Use fibreglass mat and polyester resin. Finish the surface afterwards with a thin layer of body filler. As others mentions, grind out the crack, don't be afraid to open it up say 1/4 inch, bevelling the edges, and grinding down for clearance for several layers of mat for about 6 inches wide. You will be able to figure out how thick it is when you grind open the crack and bevel that edge. Grind with a course sanding disc to make for good bite. Wear a dust mask to avoid inhaling the dust. When finished, sand it down smooth and apply thin layer of lighweight body filler and sand to perfection. Luckily the top of a jeep isn't subject to that much scrutiny. My top was in several peices and the hinges pulled out with the plates when I got it, but it looks as good as new now. Best to soak the matt strips in the resin after mixing with hardener, then apply the stips one or two at a time (you don't need to repair it in a single application of mat), brushing or rolling out any air bubbles or pockets. Remember, it's the glass that gives it the strenght, so use more glass, minimal resin to get the job done. Sand between layers to get best adhesion. It will be as good as new.
Tim
84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster