Mumps measles and other such illnesses can cause the hearing nerves to get damaged or weakened and thus the hearing goes down. In your case it seems to be the same cause and thus the hearing in the left ear has gone down. There is no medicine or surgery that can restore what is lost but if you take timely precautions you can keep the other ear safe and prevent the weak one from getting further weak.
1. Avoid loud noises, crackers, humming noises etc.
2. Avoid misuse of antibiotics
3. Avoid use of ear drops without the advise of the doctor.
4. Avoid drugs which affect the hearing. Take advise of your doctor all the time you decide to take a medicine.
5. Keep checking up your hearing every time your doctor advises you to.