Users' posts (on Stack Exchange) have been undeleted due to "vandalism". This can be when a user rage-quits the site and deletes all their posts.
A post can be rolled back when a user defaces the post, for example editing a post to contain inappropriate images.
However these instances of vandalism are mostly related to answers, because the system doesn't allow users to delete many questions. I can only recall two times when users have undeleted a question, both spawning from my question Asker deletes question after posting an answer. With the conclusion you can vote to undelete anything you want, just like the OP can vote to delete anything they want.
However, I think we should only undelete questions with answers. For example, where both 301 and I posted an answer and our time and effort was unilaterally deleted by the OP.