Try this first -
Put the battery in the charger, and then rapidly plug/unplug the charger a few times.. just move the plug back and forth fast ( like an inch stroke) to make contact with the socket. You want to keep doing this quickly so you don't give the charger a chance to "read" the pack. If your lucky, that might work. Sometimes the cells discharge so far that the charger reads them as defective when they really aren't.. This happens if you let the packs sit for a long time without charging them - so they continually discharge. The idea with this is to force a few volts into the pack, and hopefully get them going again. Any of the repair / jumpstart vids will only work on packs that discharged too far. If they have bad cells - they are done..
If the charger doesn't work - you can also take 2 good 18v packs and hook them up in series to get 36v. ( make sure you check the polarity of the pack online, and find out what side is + and - ). Hook up one lead to the dead pack, and take the other lead and tap the other terminal 5 or 6 times.. Remove, and place in charger - see if it works.
Edit - to be clear, just watch this :