If you find a bumblebee which appears to be struggling, it may be that it is just resting, particularly if the bee is a queen in early spring. If you think the bee is struggling the best thing to do is gently put the bee onto a bee-friendly flower.
If there are no bee-friendly flowers around, mix 50/50 white sugar and water to give the bumblebee a one-off energy boost, providing the carbohydrates it needs to fly. Simply offer a drop or two of sugar water up to the front end of the bee on a teaspoon or an upturned drinks cap in a sheltered place and allow the bee time to recuperate.
(It is not advisable to use brown sugar as it is harder for bees to digest and don’t give bumblebees honey as this can contain pathogens.)