If you are using an android phone and see this message:

"Can't establish a reliable data connection to the server. This could be a temporary problem or your sim card may not be provisioned for data services. If it continues call customer care."

Try checking your date and time settings. If these are incorrect, it can prevent you from logging in to your gmail account. I fixed the problem on my wife's Samsung Galaxy by doing this, when clearing the google application data did not fix it. It saved me from having to do a factory reset and losing all contacts and application data.