My iPhone text message app won't open. What is Wrong?

roy ~ Apple's iPhone User Guide says: "If your iPhone isn’t working as expected, you can try restarting it by turning it off, then turning it back on.":

Turn iPhone on or off - Apple Support

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If a simple power off and power back on does not work, the next step would be to perform a Forced Restart.

Force restart iPhone - Apple Support

Follow the instructions very carefully.