Grab a multimeter and check the pack's output: on an 18/20V pack anything near zero means the BMS has opened and the charger will blink, and under roughly 12V suggests it's too deeply discharged for a normal start. If it's at room temp, reseat it and leave it blinking on the dock for 20-30 minutes-many chargers do a low-current precharge and will switch to solid if the cells recover. If it never changes or the voltage stays near zero, skip jump-start hacks; the pack is likely done and the charger is probably fine.
