Do you see a little moon icon at the top right of the screen, or is the moon symbol white (on) in the Control Center that you drag up from the bottom of the screen? If so, that is Do Not Disturb, and it will prevent the ringer from sounding while it is enabled. This is a very common cause of your issue.
-Hmmm...maybe the ringer switch isn't seated properly to the "on" position?
-Do you have all of your data backed up to your computer or iCloud, so you could try a restore?
-Do you have an Apple Store near you that you can visit to get it checked out?
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