usually on the left edge, near the top, there is a sliding button (switch).
This controls whether the iPhone is silent (vibrates) or rings.
Look at it and make sure that you cannot see any red. Move the switch to the opposite position.
Also check the System Preferences, under Sound, for any possible problem there.
You could also try a Restore to Factory defaults, making sure you backup first.
If all else fails, take your iPhone (whatever version it is, we don't know because you didn't tell) to the Apple store.