If you have any honey (or you can make up some sugared water), place a small amount by their miuths, they wil drink this, if not too far gone and fly away. I read this somewhere. tried it and it worked! Don't know why they are dying, but this seems to happen a lot sadly.
Maybe they've reached the end of their lifespan...about 28 days for a worker bumblebee. I have them nesting in my cavity wall for the second year and I'm finding quite a few dead ones now.