Well it may be just turning off on the phone, or it may be that the WiFi you are connecting to is losing the connection (not on phones end) which would cause your WiFi to disconnect.
Try these...
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. Now, if you are still having the problem at this point, and you are sure it isn't the modem/router that is...