Airtel — lost my prepaid balance

I have lost nearly ₹300 balance from my airtel mobile ([protected]) prepaid while changing the simcard... Actually there is some problem with my sim card for last 2 weeks... We called the customer care and checked with them... They told services are normal to your number and the problem may be with your sim like that... So they asked us to change the sim card for the same number... So we changed the sim and it got activated on 6th jan.. But it showing as ₹0 balance... Kindly check it and help us to get back our balance... Thanking you...