We are currently investigating issues some players are having on the North American and European PC/Mac megaservers. We will update as new information becomes available.
What Dark Brotherhood level are you stuck at. If it's anything less then level 9, then need to do some DB daily's to get your guild level up. Get the quests from Speaker Terenus and Elam Drals in the Dark Brotherhood Sanctuary. Once you pick up the next guild level a main guild quest should open up for you so you can progress.
"We have found a cave, but I don't think there are warm fires and friendly faces inside."