iPhone stopped ringing?

Some things to check:

The ring/silent switch on the left side of the phone. See: https://support.apple.com/en-us/HT208353

Settings > Phone > Silence Unknown Callers: https://support.apple.com/en-us/HT207099

Do Not Disturb: https://support.apple.com/en-us/HT204321

If none of the above, also try restarting: https://support.apple.com/en-us/HT201559

That happens a lot -- many (maybe most) people rarely use the switch and if it gets moved accidentally it's an easy thing to forget to check.