All videos are stuck on the loading screen.
This could be caused by a number of things. First, you'll need a broadband connection with at least 500+Kbps for the best viewing experience.
If your connection speed meets the minimum requirement but you are still stuck on the loading screen, this could be caused by a firewall or internet blocker. These types of programs could block the media stream from YouTube. If you have a firewall, you might want to temporarily disable it to see if the videos load. If so, you would need to adjust the firewall settings to allow streaming media from YouTube to pass the firewall. If you are not sure on how to do this, please contact the manufacturer of your firewall.
This might also be caused by another streaming application such as Quicktime fighting for the stream. Please check streaming applications installed on your computer like RealPlayer, Quicktime, and Windows Media Player and ensure they are not enabled as the default player.
Finally, you may be trying to view a video that has been marked as "Private" by its owner. Unless you are logged in and a member of that member's "Friends" or "Family" list, you will not be able to see the video. Check another video to determine if all videos are failing to load, or just one.