As far as i know, drafts only survive for about a day.
If it's gone, i don't think there's anyway to get it back.
If you are going to work on a draft for a long time, i recommend to either write it somewhere else in the first place until it's finished, or if you prefer to write it here, copy it somewhere else regularly.
If you want to go for the copy method, i recommend first activating BBCode mode, and then copying, so everything is preserved intact.