Welcome to Android Central! Does this persist after a reboot? If so, try going to the App Manager, select the keyboard, and Clear Cache/Clear Data, then Force Stop. If that doesn't help, then see if it still happens in Safe Mode: https://support.t-mobile.com/docs/DOC-29226
Same thing happened with the wifes phone after upgrade. Tried several things and didn't help. Went into S Voice and the keyboard mysteriously reappeared, sometimes it's better to be lucky than good.