Red.es, the registry who manages the .es extension (Spain), experienced technical issues with their nameservers earlier today. Because of this several .es domain names were not or hardly accessible. The problem should be solved now. The problem would have been caused by a failed DNSSEC signing of the zone file, but no further details have been published yet.
Technical issues with the nameservers of a registry are very rare. The correct operation of the name servers is of course the core business of every registry and they do everything to get 100% uptime, but sometimes something still goes wrong. The previous large-scale DNS problems at a registry that made the news were from DENIC (the registry for the .de names), but that’s already 8 years ago. Previously, .nu, .se (Sweden) and .biz had also experienced similar problems.