@himangi are you on $15 plan with limited outgoing minutes?
Login to My Account using Incognito mode, confirm if the account status is active and if you have outgoing minutes left
if account status is suspended, make a manual payment to resume service
If you are out of minutes, buy extra minutes addon
if those are not the cause, try to reboot the phone. Also, try your phone in another area as it could be just local network issue
You might want to change the phone's Perferred network or network mode to 3G Only (WCDMA) and see if the phone can connect at all. If not, it is a local network problem