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.

Maybe her Internets not working. Does it said delivered under the message? If not then tap on the message then it will say to send message by text:)

Ask her to send you an iMessage from her iphone 5 first, then your iphone will recognize her one and the typing box will get blue. It happens to everyone when they change their phone even though they keep their SIM card. The two devices must detect one another from scratch.

I have an iphone 4s and one day the gay 3G network wouldn't work. I reset my phone and turned the internet on and off. I walked around the neighborhood trying to find signal. Then I finally called apple on another phone. They couldn't help me of course, so I handled my problem my way. Go through your entire settings and turn EVERYTHING off. Turn off your phone and turn it back on. Turn everything back on. TADA my phone came back to life and I had perfect signal. Maybe this will help.