If you find a grounded bumblebee in the winter or spring, then it is probably a queen who has been caught out in a sudden shower or cold spell! Bumblebees cannot fly if they get too cold, so the bee will need and appreciate your help. Bumblebees are very passive and will only sting if you’re actively threatening them, so don’t be scared to lend a helping hand.
Get the bee onto a piece of card or paper (or get a tiny pebble, they will probably climb on to it) and move it somewhere warmer, drier and safer (for example in a plant pot on its side, under a leaf or some other sheltered place you have). You can also feed the bee using either a 1:1 mix of sugar and water or a 30/70 mix of honey and water (sugar in preference if you have some) – just put a drop of the mix on a suitable surface within the bee’s reach but make sure not to get it on the bee. Once the bumblebee has warmed up and fed, she will most likely fly off.
By saving a queen you may have saved an entire nest.
(If the weather really is awful and not bumblebee friendly, you can keep her somewhere sheltered until it improves, but make sure to remember to provide food, and let the bumblebee out if it starts flying around)