Why isn't my iMessage working on my iPhone 5?

It's fully updated as far as iOS goes. I was on my run today and literally it just stopped working out of nowhere. Looked online for troubleshooting but Apple doesn't help AND I don't wanna screw my phone up any worse (whatever is going on) by doing something to activate iMessage again. Anybody have a proven trick that may help me get this back on track? Much appreciated!

If you have AT&T I know a lot of people that do in my town and they were all tweeting about how iMessage wasn't working, so your service provider may have just had a slight mishap! My friend that I was texting just got her iMessage up and working again though, so I hope the case is the same for you! Good luck!