How do I fix it so it will take a charge again

There is no easy way to fix them so the charger will accept them again, there is a PCB inside with a control chip that tells the charger if the battery is somehow under-volted and it stays that way even after a manual recharge of the individual cells.

One way is to use an RC-charger with low current directly on the battery poles.

Another way is to change the PCB, like this: http://www.instructables.com/id/Makita-1..., there is a number to an ebay item in comments.

I have even seen a guy who soldered all the components off the PCB and hooked the cells to the yellow connector, and them modified a charger so he could do balancing with an RC-charger.