Some reasons that an iPhone won't connect to the Internet include the airplane mode being turned on, the phone having out-of-date software or difficulty in locating or connecting to a network. In most cases, it is an easy fix to resolve these issues.
The first place to start when having connectivity issues is the settings. One should turn off the airplane mode if it is on and check that the settings allow for an Internet connection. If these settings look good, one should then check to make sure the phone has the latest software installed. Many times the software can glitch if it's out-of-date. If the iPhone has difficulty locating or connecting to a network, one should erase the network memory function, scan for available networks and log into the appropriate network. Sometimes, an old-fashioned reboot will do the trick as well.