Arduino Uno Wunderground weather station - Ethernet connection failed
I assume u used a W5100 shield. This shield is capable of having 4 connections open. If the connections are not closed for some reason the chip is unable to make another connection. 
This means after sending u need to close the connection. (else it will work 4 times). 

ethernetClient.stop()

Just before or after

Serial.println("New data uploaded");

https://www.arduino.cc/en/Reference/ClientStop