Batteries won't charge.

Could be any number of things, Jim, but we don't really have enough to go on to be able to tell you what's wrong with it.

First of all, before we can give you specific advice about, for instance, to replace the batteries, we'd need to know what vacuum you have. Manufacturer and model information is needed.

What have you checked? Does the vacuum have a separate charger? Have you verified that the charger is putting out the correct voltage? Is there some sort of indication that it's charging when you plug it in? Does it work at all on battery, even if just for a moment?

How old is it? Depending on age, it could need the batteries replaced, even if they are rechargeable; most devices use lithium ion batteries nowadays, which have a limited lifetime.

That's about all the generalities I can provide, given that you've given us a total of three words describing your problem and we don't know what vacuum you have. Please edit your question or add a comment with more details and we'll see what we can do to help.