I broke the main steam doing LST

Only chance would be to graft it back on. Get a sharp knife and cleanly cut the stem then quickly after you want to cleanly cut the "stump" sticking out of the ground. Once both are cut stick the cut ends together and wrap with medical tape. Then add a stick to brace the break and wrap again. After 2-3 days you can inspect and unwrap. But this was at least 1h ago, Not sure how long you would have to do the above and be successful. If done in the first 10 min you would have a good chance.