I feel numb on my hand and fingers?

See a doctor. Numbness in the hand an fingers could be the sign of nerve compression anywhere along its course- from the neck to the hand. Symptoms can resolve spontaneously after changing position or with time. If they do not, you should see your doctor.
Many things. This could be from many things: nerve inpingement, nerve damage, sodium or electolyte issues, glucose issues, repetitive injury, carpal tunnel issues, other neuromuscular disorders or even a sign of a heart attack. If this persists/worsens seek attention from a physician for further eval and management.