iphone needs to cool down

First of all, you may try to reset its settings and see if this helps.

Try to reset your device. This won't delete your data stored on the phone.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

If this does not help, I believe you need to have your device checked by local Apple Store specialists. They will either repair it, or give you a new phone.

Were you somewhere using the phone that the temperature exceeded the operating limits?

And did the iPhone feel hot when you picked it up?

If this occurred spontaneously and the phone was hot it could be a battery malfunction, make a Genius Bar appointment and have them check the battery. If it wasn't hot try Restoring your iPhone from your backup in iTunes. If that does not work use your warranty to get it replaced.