It might be worth attempting a restart of the iPad, and then a reset, should restarting fail to resolve the issue.
If the keyboard still won't appear, I would recommend restoring the device.
All the best,
In instances where I've lost the virtual keyboard, it was because I had enabled Bluetooth to use an external keyboard, and I had either turned off the external keyboard or moved the iPad out of range of the keyboard. In these cases, going to Settings and turning off Bluetooth made the virtual keyboard come back.
Hope this helps,