Louder ringer volume

I assume you tried an alternate ringtone—

You can try User troubleshooting:

Follow the 3R's- if a restart does not solve your problem proceed

1. restart, ie power off/on,

2. reset http://support.apple.com/kb/HT1430,

3. restore from backup, restore as new device. http://support.apple.com/kb/HT1414

If you think it is a larger problem than just this one isolated incidence,

you contact Apple Support, make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for it to be inspected and to establish your available service options:

Contact - Official Apple Support

https://twitter.com/applesupport

http://www.apple.com/retail/geniusbar/

https://locate.apple.com/

Or your store/carrier where you made the purchase

Is this a default built in ringtone or a custom one you loaded in?

Use tones and ringtones with your iPhone, iPad, or iPod touch - Apple Support