I was playing on Llane when it went down without explanation around 12:30 p.m. EST. It looks like it wasn't just my realm. According to this announcement, 36 servers in the Vindication and Ruin battlegroups are currently down with no notice of when they might come back up.

We're aware that players are currently unable to log in to the following realms and we're in the process of investigating. We will provide further updates when they are available.

The full list of affected servers is after the jump.

UPDATE:  Around 3:44 p.m. EST, Bornakk posted that the issues have been resolved and the servers are currently being brought back online and should be playable soon.

Aggramar
Arthas
Azgalor
Bleeding Hollow
Bloodhoof
Burning Blade
Burning Legion
Drakkari
Durotan
Earthen Ring
Elune
Eonar
Eredar
Gilneas
Gorefiend
Kargath
Laughing Skull
Lightning's Blade
Llane
Lothar
Madoran
Magtheridon
Malygos
Mannoroth
Medivh
Quel'Thalas
Ragnaros
Shadowmoon
Shattered Hand
Skullcrusher
Stormrage
Thunderhorn
Thunderlord
Trollbane
Warsong
Zul'jin