If the plant stalk is totally severed, I would start again. If it is still connected, just get a splint (like a wooden skewer) and use that to support the break, while wrapping a piece of cloth around to hold the splint on. MJ plants are quite capable of mending themselves - depending on the severity, sometimes within three days. Once healed, you will generally notice a "lump" in the stalk of the area where it broke.
Good luck.