This is a huge subject , and the following site gives you the main causes:

• Deafness can be inherited: if one or both parents or a relative is born deaf, there is a higher risk that a child will be born deaf.
• Hearing impairment may also be caused before or during birth for several reasons. These include:
  • Premature birth;
  • Conditions during birth in which a baby lacks enough oxygen to breathe;
  • Rubella, syphilis or certain other infections in a woman during pregnancy;
  • The use of ototoxic drugs - a group of more than 130 drugs (such as the antibiotic gentamicin) that can cause damage to the inner ear if incorrectly given - during pregnancy;
  • Jaundice, which can damage the hearing nerve in a newborn baby.
• Infectious diseases such as meningitis, measles, mumps and chronic ear infections can lead to hearing impairment, mostly in childhood, but also later in life.
• The use of ototoxic drugs at any age, including some antibiotic and anti-malarial drugs, can cause damage to the cochlea (the hearing organ in the inner ear).
• Head injury or injury to the ear can cause hearing impairment.
• Wax or foreign bodies blocking the ear canal can cause hearing loss at any age.
• Excessive noise, including working with noisy machinery, exposure to loud music or other loud noises, such as gunfire or explosions, can damage the inner ear and weaken hearing ability.
• As people age, accumulated exposure to noise and other factors may lead to hearing impairment or deafness.

http://www.who.int/mediacentre/factsheets/fs300/en/