charged batteries the wrong way

I bought a cordless hedge trimmer and it has a rechargeable battery pack that you remove and put in the charger. The second time I charged it I think I may have put it in the wrong way (negative to positive), because the battery pack got very hot and when I put it in the hedge trimmer, the appliance didn't work. Will it be safe to try to charge the batteries again, correcty this time? Or have I ruined the batteries forever?