First, try refreshing the page, usually that will fix it. If that doesn’t work, you could try and launch a different browser. If that doesn't work, make sure you have a stable internet connection.
If you're using wireless, try and move closer to the wireless access point. Another thing that can help is to make sure you're not streaming internet tv, videoconferencing, downloading large files or anything else that might take larger amounts of bandwidth. If all else fails, try and restart your computer.