How come my HTC EVO Shift 4G will not let me text?

I just got the shift from Sprint and I love it. I was texting the first three days then I got an update yesterday. Now I cannot text on the messaging application that comes with the phone. Everything else works though. I can go on the internet receive texts call and receive calls but I cannot text. I'm not sure if this helps but when ever I attempt to text I get an En in my notification bar in the top right corner. PLEASE HELP...