I cannot send text messages, what is wrong with my phone?

Today morning I could text and all, there was no trouble but then all of a sudden it stopped working. I received a text and when it was time to reply back, it takes a really long time to send and after some seconds it ends with ’ sending message failed’ I have switched off the phone, taken out the sim card and battery. Put them back together but still it didn’t work. I also deleted all the msgs in my ‘sentbox’

Help