Broccoli (especially the calabrese type) are a bit fussy and tend to produce in a set number of days. I had to keep mine in small pots for longer than I wanted before planting out because of the weather and many of them have done just the same as yours. If they get dry at any point, that tends to ruin them, too. If they are the type which produce side shoots, cut the main head (now flowers!) off and they may still give some useable crop from those side shoots.
