What took place inside the motor and why isn't it working anymore? I familiar with basic electricity and circuits, but can somebody explain it in easy language.
First, don't do that. Ever. Seriously, you can injure yourself or others or start a fire.
Second, DC motors are typically rated for around 10 V. That means that they have fairly low resistances. Your AC outlet probably operates at about 120 V if you are in many countries or even more in some countries. That means you are overloading the voltage capacity by a factor of 12 or so, approximately 1200% above the rated voltage.
Third, voltage is roughly proportional to current by Ohm’s law, so you also have a factor of 12 more current than designed.
Fourth, power is voltage times current. So your little DC motor was receiving about 144 times its designed amount of power. That means that the coils and other internal components were heating up 14400% faster than intended. So of course “sparking happened”, that was some part of your motor melting due to the excessive amount of power. Once the part melted it probably formed a short circuit, further reducing the resistance and increasing the power dissipation. That would continue until it caught on fire, melted to an open circuit, or tripped the circuit breaker.
Fifth, see first: don't do that. Electricity is dangerous. It is not a toy, so don’t play with it. You seem like a smart kid, so let’s make sure you live to become a smart adult.
It is not clear from the question as to the output voltage of the AC power outlet but even it was $10\, \rm V$ there is good reason as to why the dc motor would overheat and be render useless in the process.
When a dc motor is operating correctly the current through the motor does not only depend on the supply dc voltage and the resistance of the coil as explained here..
The rotating coil generates an (back) emf in opposition to the applied voltage which helps limit the current.
Indeed if an dc motor was “ideal” the coil in a dc motor would rotate at a constant speed and the current would be zero.
When an ac supply is connected to a dc motor the coil does not rotate and so there is a very large current passing through the motor which means that there is a large ohmic heating effect and this could result in the coil burning out - part of the coil melts and this results in an open circuit.
The reason why the coil does not rotate is due to the fact that the current though the coil of the motor does not reverse synchronously with the position of the coil relative to the magnetic field.
In a simple dc motor the current in the coil reverses every half a revolution which produces continuous rotary motion.
The split ring commutator performs this function.
Applying an ac voltage which reverse many times per second to a dc motor means that before the coil has a chance to rotate in a particular direction the current is reversed and the coil tries to move in the opposite direction.
The best the coil can do is to rapidly vibrate about a mean position, so in effect the coil is not rotating, little back emf is created and so the current though the coil is dangerously large.