For a couple of days now many users of a Chromecast haven't been able to stream anything. The problem is acknowledged by and seems wide-spread.

On their support website, Google reported Thursday night that a fix is coming up in the form of an automatic update for Chromecast devices. The issues got awareness after website Tweakers in the Netherlands posted about it.
 

The solution should be available at the time of writing. Google encourages users to restart their Chromecast if they can not yet Cast. Google, however, did not give any further information about the problem. On Wednesday, Chromecast users worldwide complained that they could not Cast to their device. The Chromecast was seen by smartphones and tablets via the Home app, but did not appear as an available Cast device if for example YouTube or Netflix was started.

Google releases fix for streaming problems on Chromecast