IPhone texts have gone from blue to green, imesage not working anymore

I have an iPhone 4s and I have had no problems with it far so, last night I was texting my boyfriend who also has an iPhone 4s and our texts have always been blue.
Last night we were just texting and all of a sudden none of my texts would send so i turned me phone on and off and when i turned it back on every text that was sent was green.
Now whenever I text someone who has an iPhone none are the texts are blue, they're green.

I haven't changed anything in my setting/I don't know if there's anything in the setting to help?

Does anyone know what has happened and how I can fix it!

Sometimes iMessage turns itself off. Just go to settings, iMessages, & switch it to on. If it didn't turn itself off then call apple.