Hi guys
You forgot your Airtel PUK code or u unknowingly made that box to appear no worries
here is what u have to do

For getting PUK code of a Airtel number, send PUK<space>PhoneNumber to toll free number 52212. You can make use of any other Airtel number to send this message since your SIM might have been blocked already.

If the above method doesn’t works or your service provider is not Airtel, call to customer care using any other mobile phone which uses the same service provider. They will ask some details about you to make sure you are the owner of the SIM card. Questions might include your billing address, email ID, numbers you recently called or messaged etc. They might even ask you number printed on the SIM card, so be prepared with all details before calling them. Once you get the PUK code, enter it into the phone & your phone will return to normal.
Read note for method.

[b]Note:
1) Make sure you type PUK in Capital letters.
2) Provide space after PUK.
3) Type 10 digit mobile number.
4) Make sure you use anohter Airtel service phone to send sms to 52212.
eg:
PUK 1234567890 and send it to 52212.
5) You will receive PUK no from service provider as reply.
6) Type PUK no and enter New PIN code as 0000 or 1234


Note: Make sure not to enter the PUK code your self after 10 incorrect attempts the Sim will be permanently blocked and then u'll need to get a new one!