Usually just removing and replacing the battery will do the trick. You may need to do it a few times before the light stays solid red but that hasn’t happened with mine... yet. If the light continues to flash it could be either the battery or the charger that’s at fault. If you have another battery you can see which is at fault. If it’s the battery, it could be it has discharged to zero volts, in which case it’s toast. However, there may be a way to bring it back to life if it has not gone completely dead (zero volts)... Place another battery across the suspect battery in parallel for a minute or two. The battery may have discharged not to zero but to a point that will not allow the charger to work, It’s hard to connect the batteries together with the way the contacts are on the battery but if you have a steady hand you can pull it off. While the batteries are connected in parallel (plus to plus, minus to minus) feel if it is getting hot. If it is remove it to preferably outside where if it does continue to heat it won’t catch anything on fire. Then dispose of the suspect battery properly. The idea is to increase the charge in the suspect battery to a point above where the charger will activate and start charging the battery. Monitor that battery while charging, it’s still suspect... checking for excesive heat just to be safe. Lithium ion batteries need to be respected because of fire and explosion but as long as you continue to feel no rapid buildup of heat, you should be ok. To be extra safe, only try and recover a suspect battery outside your house or garage... and pop it out of the charger if you feel heat and put some distance between you and the battery... toss it out onto the lawn and give it 15 minutes. Then dispose of it as directed. It’s bad.