Imessage suddenly changed to Text message?

I have an Iphone 4 and my friend has an Iphone also, but I don't know the brand. I was sending him a message and right in the middle of sending, the "Delivered" icon that would usually appear didn't show up so I thought it was a delay. I turned my phone on and off thinking it would be fixed but found that his imessage had switched off and my text had been sent as a Text instead. All my other contacts who have Iphones still allow imessage, except for my friend's phone that spontaneously changed to Text Message mode. Is there something wrong with my Iphone or is something wrong with his?

There are settings on imsg where you can say send as text if imsg fails. You can turn that on or off. Also obviously the receiver needs to have imsg turned on.