A faulty foot pedal, locked drive gears, wiring failure or bad drive motor will prevent the sewing machine motor from running.
Unplug the sewing machine and check the wiring connections between the foot pedal and the motor. Reconnect any loose wires and repair any broken wires. Use a multimeter to check for continuity through the foot pedal switch (with the sewing machine still unplugged). Replace the foot pedal if the meter measures no continuity through the foot pedal switch with the pedal depressed.
Spin the hand wheel to check for drive system binding. If you can turn the hand wheel, clean and lubricate the sewing machine to free up the drive gears. If the sewing machine motor buzzes but doesn't run, a motor bearing may be locked up. Replace the sewing machine motor if it buzzes but doesn't turn.
If the sewing machine motor doesn't make any noise when activated, then an internal wiring failure in the motor may be preventing the motor from running. Replace the motor if it won't make any noise and won't run when activated.