I lost all of my cards whilst playing Warlock in a ranked game (started drawing dead - only just lost / more suicide than anything)... well when I went to edit my warlock cards, I couldn't anymore. Is this part of the gameplay or a glitch?
There are no restrictions on creating, editing or deleting decks. You can never lose cards through game play, only through disenchanting in "My Collections".
If you experience an interface bug, and restarting the client doesn't resolve the issue - use the bug report forum.
seems your account was logged somewhere else exactly when you started playing and someone disenchanted your cards or due to a server bug you got your account reset. account resets are possible but only doable by blizzard employees