A few things to check. Does the motor spin freely? If you bumbed the plane the motor may have been pushed back and could be dragging on some part of the fuse or a motor bearing could have gone bad. Both of these would cause higher current draw and could be overheating the ESC, motor and could puff the battery. Second how many amps do you have the charger set for - is it set for Li Poly batteries? Sometimes setting on the charger get changed because of the hidden key board functions (i.e. if you hold down a botton for more than x secs and it brings up options that can change the original setting). Get the charger manual and make sure it is set for the type and size pack you are charging, go thought all the set-up steps to make sure. If you have a DC amp meter (Sear has a $39 clamp on type make sure it is for DC amps) or can borrow one, check the motor amps with the flight prop at low throttle then at full throttle, it should be no more then about 25 amps static on the ground at full throttle. If you are pulling more then the prop is loading the motor to much for the ESC to handle and you can either up in a larger ESC say a Castle Creation 35 (if you do this you need to make sure the amps are less than 40) or reduce the prop size until the amps are lower then 25.

Best, Dennis