Bumblebees eat nectar and pollen. Source
Gymnosperms are plants that have small flowers. The flowers have no special coloration or smell. If you don't count them as flowering plants (they technically have flowers, but not what we think of as flowers), gymnosperms will still be around. The flowers release large amounts of pollen into the air in the hopes that some of it will land on the flower of a plant of the same species. If these plants still exist, bumblebees would collect pollen from them as a food source. The bumblebees would collect pollen from the plants and off of any surfaces where the pollen would have landed. With this behavior, I think bumblebees would still be able to survive. Without flowering plants, gymnosperms would have less competition and wold increase in number.
In a world without flowering plants, you will still have conifers. Conifers don't have flowers. Like gymnosperms, their pollen is also blown by the wind and ends up in large amounts on the ground. The bumblebees can eat the pollen on the trees and anywhere pollen has collected. While conifers are not as common as gymnosperms or angiosperms , they would provide a good food source for bumblebees. While the bumblebees would decline in numbers, they could still survive the loss of angiosperms and gymnosperms.
but they need nectar. IOW, pollen without nectar means no bumblebees.
Thanks to @RonJohn for pointing out that they would need nectar (emphasis theirs). I looked it up and found this article, which says that nectar is a carbohydrate. It doesn't seem to have any essential proteins or fats like pollen does (essentially empty calories), so I would guess that with a little adaptation, the bumblebees could survive without any nectar. That said, if angiosperms all die out at once, that will probably kill off all the bumblebees.