If it's the soft rubber seal, try something like Shoe Goo. It will mend the two ends together. It will seal them back together so the rubber is still pliable and will still seal. Be careful not to create a large lump at the glue site or you'll create an area where water can leak in (use only as much as you need to get the pieces to stick together). Also, you'll need to leave the hatch open until it is completely cured or it will leave a deformation in the rubber and could possibly adhere to the vehicle body which could rip apart the seal even worse the next time the hatch is opened.