Paypal : The underlying connection was closed: The connection was closed unexpectedly

I am using NVP / SOAP APIs. I use paypal sandbox. The Url which I use is :https://api-3t.sandbox.paypal.com/nvp The code was working fine but all of a sudden it stopped working. It gives the following error "The underlying connection was closed: The connection was closed unexpectedly.". Is there anything that is changed in paypal or am I doing something wrong. If you need more details please let me know. Any help will be greatly appreciated