What's wrong with my phone?

If the phone is still under warranty, by all means get a replacement. there are many trouble shooting methods, But based on the conditions you give, and not knowing if the device has been tampered with (jail broken)or altered in any way, there's not many alterntives other than taking it to a (good certified technician). you may want to get an estimate before fixing, because the repair just might be higher than a new phone. but keep in mind, if the device has been altered by any means other than the original manufactures specs.(jbreak,ect.) the issuing service provider and manufacture Will Not repair or replace the device.period.