DIAGNOSIS When the drill only runs on high speed (the "2" setting), the two-speed gear selector switch is the prime suspect. It's a physical plastic switch that slides a gear, and it can get gummed up with dust or the internal contacts can fail.
FIX First, work the speed selector switch back and forth vigorously between the "1" and "2" positions about ten times. Sometimes this is enough to scrape off corrosion or dislodge debris. Try the drill again in speed "1". If it's still dead, you'll likely need to open the housing to clean or replace the switch.
IF THAT DIDN'T WORK If toggling doesn't help, there could be a broken wire or connection inside the drill leading from the switch to the motor. This requires opening the drill's clamshell housing to inspect. Look for any obvious wires that have come unsoldered or disconnected near the switch assembly.
HEADS UP Running only on high speed isn't the end of the world—you just lose your low-speed, high-torque setting for driving screws. If you open the drill, be very careful with the plastic tabs on the housing; they're easy to snap.