This can happen when an external (Bluetooth) keyboard is connected to your iPad. If so, disconnect it.
If you don't have an external keyboard, it is caused by an internal issue in the iPad that a restart fixes.
Keyboard not appearing on my iPad
Disconnect any Bluetooth keyboard; if there isn't one, restart the iPad.
Frequently asked questions
Why doesn't the on-screen keyboard appear on my iPad?
Usually because an external Bluetooth keyboard is connected. Disconnect it. If there isn't one, restart the iPad.
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