That is one wierd error and one certainly worthy of contacting tech support for.
Sounds like they crashed somehow. Do you remember when you last accessed them? Where you accessing them when you dc'd? I remember retainers having that bug once, where if you entered a duty while a retainer window was open led you to be unable to access them again.

Perhaps they are too busy lounging in a beach though. You'll need to punish them (on a less serious note)