My left foot started hurting like something smashed it and it keeps going numb. All I was doing was sitting down. What causes this and what can it be?

Sitting and nerves. This sounds like nerve pain. Neurological problems like the spinal nerves getting compressed in the lumbo-sacral region or from pressure on the sciatic notch can cause this. Also sitting can cause pressure on the leg nerves. try getting up and moving around. Problems of the feet like nerve intrapment or planter fasciitis may cause this. Have a doc exam.