If you updated to the most recent version of Google Web Accelerator, it screws up most YouTube videos and you get that error or just a continual attempting to load. You would think that Google would resolve this considering that they also own YouTube.
Disable or uninstall Google Web Accelerator. If you don't have GWA, try installing the latest version of Flash or Java RTE. Also, try running it in Firefox and Opera. Finally, clear your cache/cookies/etc. One of these solutions should solve your problem.