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What is the model and make of your phone ?
Which model of phone is this ? Samsung, Sony, Apple iPhone etc
First make sure you didn't turn on the silent/mute switch. If it is turned on (you will see red color on the switch if it is on) then turn it off. It is on the left, above the volume buttons.
If that didn't work, then go to Settings > General > Reset and tap Reset All Settings. This will reset some settings like WiFi, Ringtone settings, wallpaper etc but won't delete anything from your iPhone.
Did that work ? Please let me know if that didn't work, so we can continue troubleshooting.