This usually happens if YouTube is performing maintenance on their servers.
You can continue to play videos which are on a different server from those that are currently being worked on or those servers which have be brought back online because their maintenance has been completed. However, there's no way to tell until you try.
Just wait it out and you can watch the video successfully have the maintenance period has passed.
Keep an eye out for the notices that are prominently displayed when YouTube is performing maintenance on their servers. This generally shows how long is left for everything to complete.