Why does my keypad fails abruptly?
I had similar problems with my  new Samsung Galaxy Tab 3. When I wanted to write a post 
for a forum I got the writing window and the keyboard but when I typed three letters 
the keyboard vanished and this problem continued.
Later I discovered how to really shut down the tablet by pressing the switch-off a bit longer.
Then I got a message: Switch off the tablet? I clicked okay. When I turned on 
my tablet again after some time the trouble with writing was gone.