Why won't my iPhone 5 send iMessages anymore?

It's an iPhone 5 I got in February and it won't send iMessages anymore and yes I'm sure I'm sending the messages to an iPhone and my cellular data is on. Please help! Thanks!

It might have something to do with your messages settings.

Go to settings - messages

Make sure iMessage is set to on.

If it's still not working, it may be due to the iMessage server being down OR the person's iPhone you're sending to has not enabled iMessage.