Arduino Uno Won't Power Up

I was using my Arduino Uno with a DC motor shield and it was working good, but then I heard this spark, and now the Arduino won't power up when connected to the motor shield, but it will power up when it's not connected to the motor shield. What is the problem?

At 5 volts, you could never hear a spark. You heard an overheated component dying. Get your magnifying glass out and examine every component on the boards and see if you can spot the damaged one.