Somewhere in your phone settings is a number to which all SMS text messages are sent for onward distribution to the recipient. This is the SMS message centre number. Where this is configured depends on your specific make and model of handset. If this number is not configured or is wrong then text messageing will not work. Your mobile phone operator can provide you with the correct message centre number. It may even be on their website. Did text messaging work before and, if so, what changed between then and now that it has stopped working?