Why won't't my messages go back to iMessage?

My friend and i both have iPhone 4s', and I sent her a text message a few days ago. But even when we both have wi-fi it still sends as a text message. How can i put it back to iMessage?

Go to settings > messages > turn off iMessage then turn it back on
Then go back to your texts with her, if the send button is blue then try it out. If not try turning your phone off and back on. Have your friend do the same