If you see a bee that’s just crawling around or sitting and not responding to you getting close it might just be low on energy.

To help the bee, mix as much sugar as you can get to dissolve in a spoonful of water. Once it’s well mixed put drops of it in front of the bee where it can reach with it’s proboscis. I’ve found putting the mixture on a leaf and poking that under the bee’s face helps too. It might take a bit of persistence, but once it works out what it is it’ll drink it’s fill. You haven’t seen an excited bee 'til you’ve seen one fly off after that!

It’s not particularly nutritious, more akin to an energy drink than a meal, but it’ll give them enough of a boost to get to a real flower.