I found the cause and solution to this issue as it was happening to me as well.
Keyboard input not working - BlackBerry Support Community Forums
That makes sense because the last thing I typed was, 4th! This is a bug. BB must address this.
To cause the issue:
Ensure Settings > Language and Input > Spell Check > Ignore Words with Numbers is set to Off.
Basically it is caused by typing a string like "19th" in a text message (or any message?).
So you need to set Ignore "Words with Numbers" to "on" which will resolve the issue.
Cheers