It's crippling to see the iPhone keyboard disappear when you want it. Some bug can keep the iPhone keypad invisible – and just when you need it, you might see that it's missing.
From what I've gathered, the problem can occur due to a variety of reasons most notably three: the iPhone is still connected to some Bluetooth keypad or there's some problem with an app or there's some bug in the iPhone or there's some issue with a jailbreak tweak (if you've jailbroken your iPhone).
Here's what you have to do to fix when the iPhone keypad is invisible or Missing:
1. Do a hard reset
The first thing that fixes this issue in almost all the cases is doing a hard reset. A hard reset is one where you press the home and power button till the screen goes blank. Then press the power button again to restart the iPhone.
Check to see if the keypad appears after the hard reset.
2. Switch off Bluetooth. May be Reboot once.
If you connected your iPhone to some Bluetooth device – notably the keypad – and then did not disconnect it, there's a strong chance that the iPhone is still thinking that it shouldn't show the keypad.
The one way to fix this is that you switch off the Bluetooth. May be reboot your iPhone once too just to be sure. Does this fix the issue?
3. Check for jailbreak tweak updates.
This thing happened recently. If you have pending updates for some of the jailbreak tweaks, there's going to be some kind of a buggy performance. This, occasionally, can affect the keypad.
The easiest way to fix is to update the jailbreak tweaks.
As a final step, you've got to restore your iPhone. A backup would be good. Here's hoping things don't go this far.