Can you try to call the API from the command line using wget or wget equivalent?
wget https://poloniex.com/public?command=returnCurrencies
What do you get?
This is the result:
http://imgur.com/a/XS0XPit said "Forbidden"

Maybe the API blacklisted your IP? Did you access it with other trading bots? Or did you try to talk to it with own scripts?
It could resolve itself after some time (timeout). Can you try again over another connection?