On my iPhone 4S, I can't reply to any text messages

I've only had this iPhone since November, but whenever I go to try to tap reply to a text message, it won't let me, so I spend forever repeatedly tapping on the bar, until I give up and have to go to my contacts and select "message" just so I could type. What is wrong with my iPhone? Should I go to Rogers or send it back to Apple or is this a software issue?

I have a iPhone 4S too! And, when I did that too me I turned it off and it worked fine!

Reboot it, never heard of that happening so that will reset it.