My iPhone won't let me do iMessages, only SMS?

So the other day, my iPhone randomly decided to not send out any iMessages, only text messages. I don't understand why, maybe I messed with a setting? I don't know. Can anybody help me. A lot of my friends have iMessage and I can't talk to them because of it. Normally if I'm texting an iPhone it will be blue for IMessage and if it's a regular cell it will be green. Now they are all green and I can't message any iPod or anything. I don't get it.

Stettings>Messages>iMessage (on)

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