NOTE: If none of the following work to resolve the problem, and if the phone is under warranty, I would contact the manufacturer.
1. Try and restart the phone. Sometimes it is that simple.
2. Update the firmware and any apps that need updating on the phone.
3. If the problem persists, check your time and date on the phone to make sure it is correct.
4. Should none of the former work...
a. Clear out the WiFi data on the phone.
b. Reboot the modem/router.
c. Once the modem/router is fully 'up' again, reboot the phone and re-input the WiFi data.
5. Check your internet connection. It may be what is going in and out rather than the phone.
6. Make sure you have "Data" turned off on the device when using "WiFi" (that is if...