Messages Suddenly Stopped Sending. Can Still Receive

Just as the title says, earlier this morning I just suddenly stopped being able to send messages. I'm still receiving all texts but nothing goes through. No updates had been installed at this time.

I've been using GO SMS and uninstalled that recently. The stock messaging service doesn't work, either. Every time I try sending a message I get an error stating:

"The application Messaging (process com.android.mms) has stopped unexpectedly. Please try again.

Any suggestions? I can't fathom why all of a sudden this would just stop working like it did. Hadn't been dropped or anything.. was just sitting on my desk.