Our task list is gone with all collected time sheets. Is it possible to restore it?

Here are steps - what we have done

• We created a task list for our team of developers named "General".  we used this list for 3 months.
• We decided to use milestone features and we created a milestone named "Sprint 4" and using "Associate Task List" added the task list "General".
• we can see the list of task using link "Task" and also we can see the same task list using link "Milestones"
• We decided to move everything back how it was. We deleted the task list "General" from the milestone expecting that the task list will be just removed from the milestone.
• It was a surprise that all tasks in the task list were gone completely including open tasks and closed tasks.

We use "Express Plan".