How to go from a normal text message to an iMessage on an iPhone 4s?

I have been using my iPhone 4s to iMessage but for some reason recently its been reverting to normal texting. I ave tried to start a new message all together, but it still goes back to normal messaging. The numbers i'm trying to iMessage are compatible so i know thats not the problem.

Go to:

Settings>Messages… And make sure your iMessage feature did not switch off by accident. This is the only thing I can think of.

Also, slight chance, but still a chance… That your contact switched from an iPhone to something else and kept the same number. If so, that could be it too.