You could have an issue with your internet connection, which then leads to the movie buffering as you try to stream. The Netflix website may also be experiencing issues, in which case you may need to contact Netflix customer support to have the problem solved.
It's most likely your Internet connection. You can goto speedtest.net and see if you are getting close to the download speed you are supposed to be getting. If not call your ISP and have your line checked. Upgrading to a faster Internet connection may fix it.