Airtel — unable to send text message

For some reasons unknown i am unable to send text messages from my airtel mobile number. I checked with my balance amount and tried to solve the issue from all the possible ways from my side but unable to. Last i remember sending a text message was couple weeks back, but not exactly sure what happened after that.

I even tried to contact customer care through 121, but god knows what option to select to talk to a person with whom i can discuss the issue and get it solved.