DIAGNOSIS This is often a corroded battery terminal connection or a tripped internal protection circuit in the drill from a minor voltage spike.
FIX First, perform a hard reset. Remove the battery. Hold the drill's trigger down for a full 30 seconds to discharge any capacitors. Then, firmly reinsert the battery. Try the trigger. This often wakes up the electronics.
IF THAT DIDN'T WORK Check for green or white corrosion on the battery contacts inside the drill's port. Clean them thoroughly with isopropyl alcohol and a stiff brush. Corrosion creates an insulating layer that blocks power.
HEADS UP If the drill was stored in a damp place like a garage, moisture may have damaged the main control board. In that case, it's often more economical to replace the tool than to repair it.