Airtel — my airtel number got deactivated

I was using airtel prepaid sim. Now it has been more than 3 months i was not using that and balance was also not there. Today i wanted to use that number but it is showing no service. If i call to that number it says that number is not correct. I think that sim got deactivated and i want to reactivate it. How to reactivate it. For many important things i have given that contact number. So it necessary for me to reactivate it. I need help in this.