What should i do if my imessages won't send?

Hi, i have an iphone 4 and have recently updated to IOS 7. I seems that every time to go to send an imessage to one of my friends who also has imessage it won't send, and they can't send imessages to me. I have gone to settings and double checked that my imessage is turned on.
I have tried all of the dumb people stuff like turning my phone on and off again, and turning my imessge on a and off again, but still nothing happens. I'm still able to type a message and it begins to send but won't deliver.

You can check if your iMessage settings are correctly that would be found in messages. Or you can restart your device

Tap the red mark to send again and it will go

  • Why won't my iPhone send iMessages? I have an iPhone 4S and my friend has the 5. She kept the same sim, so when she got her 5, the old messages from her blackberry weren't deleted. But now, our messages aren't sent through iMessage and its really annoying. Any ideas how this could be put right? All my other friends that I message are sent on iMessage, just not her.
  • Why won't my iPhone send iMessages to other iPhones? I can use iMessage on my iphone4 with no problems at all to iPods, but not to my partners iphone4, it always defaults to SMS. Its frustrating, because we live in an area with poor mobile coverage, so if I need to message him urgently, iMessage in theory should be the best way of doing it. Does anyone know how I can alter this please?
  • Why won't my iPhone send iMessages? - 1 So ever since I updated my iPhone 4s to iOS7, I have only been able to send iMessages to a select few people. I'm doing an exchange this year, so I need to text my exchange student, who lives in Spain. It costs $0.25 per text, which adds up and I liked being able to do it for free with iMessage. I used to be able to use iMessage with her. I tried Resetting my Network Connection already, but it doesn't work. What can I do?
  • Why won't my iPhone 6s Plus send iMessages and gets no notifications for messages? IMessage and notifications are on but it's still broken. I tried resetting my phone like its brand new and updated my phone and pretty much everything else and still nothing. Any ideas? It stopped working about two days ago