IPhone 4 message button turned gray?

When you open your messages and go to one of your conversations, in the corner, the word "Messages" is blue. It turned gray and I don't know why. How can I fix it?

Restart your iPhone. Sometimes, restart is the efficient but easy solution to fix iPhone problem. That is because, when you reboot iPhone, all the program will reboot then all the problem will be shut down.

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