The battery pack isn't making proper contact or is dead. First, remove and firmly re-seat the battery pack. You should hear a click. If it still doesn't start, place the battery on the charger. The charger LED should light up. If the LED doesn't come on, the charger isn't plugged into a working outlet. Use a different outlet.
If that didn't work, the battery pack could be at its thermal limit or the tool's switch is faulty. Let the battery cool if it's hot to the touch. A battery that has reached its maximum thermal limit will shut the tool off abruptly and needs to cool down. If the battery is cool and charged but the drill is dead, the switch is defective. Any power tool that cannot be controlled with the switch is dangerous and must be repaired by a qualified person.
Heads up: Always ensure the switch is in the off position before inserting or removing the battery. Carrying the tool with your finger on the switch invites accidents.