When-ever you snap a stem and it doesn't break off totally, you can fix it with tape and a splint. After a week or so it will form a knuckle and be fine. I have even done this with buds that were so heavy they snaped at the stem. They were five weeks into bud and after splinting them still grew like nothing happened and were just as large as the buds that didn't get snapped. If it totally breaks off then I guess there isn't much you can do. The stem only needs an eigth of the stock to supply enough nutrients to the bud or leaves, so even a stem almost broke off totally will usually heal just fine. Hope this helps.
TGT