If you can't send sms from your mobile phone may be your mobile sms settings are the cause.
each operator have thier own sms service center no's though which your message will be passed to recipient. below are the sms center no of differnt mobile telcom operators. please check it once if you unable to send sms from your mobile.. if you find other than these no then change it to corresponding operator no....enjoy.
if you still unable to send then contact to your service customer care no.and ask them
BSNL Message Centre Number: +919440099997 (for old Numbers), +919442099997 (for New Numbers),
!dea Message Centre Number: +919847099996
AirTel Message Centre Number +919895051914
vodafone Message Centre Number: +919846000040
Reliance GSM Message Centre Number: +919020000500
AIRCEL Message Centre Number: +919809099060
TATA DOCOMO Message Centre Number: +919032055002
uninor Message Centre Number: +919084051550