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If you have Do Not Disturb or another Focus profile activated, try disabling it.

You can do this in the Control Center. Pull the Control Center down from the top right corner. Find the Focus Button, just below Screen Rotate Lock and Screen Mirror, tap and deactivate the active profile.

If Do Not Disturb or another Focus profile activates automatically, then a Schedule is active.

To disable or delete this schedule, go to:

Settings → Focus → Select Profile → find SET A SCHEDULE → Tap into the time → tap Delete Schedule or just deactivate.

You can read more about how to use Focus here:

Use Focus on your iPhone or iPad - Apple Support