Make sure you click in a text entry box. If that doesn't do it, try swiping up from the bottom of the screen (around the center). This should bring the screen up. Alternatively, try doing a hard reset by pulling the battery out for a minimum of 30 seconds (to be safe, wait over a minute), then reinsert the battery. If this starts becoming an issue, you can always take your phone in and they can check if any of the settings are off.
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