My Cannondale failed at the weld about 8 or 9 months after i bought it, but they have a lifetime warrently on the frame, so i took it back to the bikeshop from where i bought it.
the frame was replaced under warrenty, and so far no further snappage has occured and i have a newer frame to sell on 🙂
so take it to the shop and see what they can do – it will mean you’ll be without a bike for ages, but it will be replaced at no cost to you.
the shop will strip the bike, send it to the manufacturer, who will scan the bike, look at the welds, provided there was a weakness there (which sounds likley)they’ll replace the frame and return to the shop.
(Edit; replace ‘will’ with ‘should’)