After a long period a post is deleted, is it still possible to edit the deleted post?
In this FAQ answer, it says
Self-deleted posts can be viewed and undeleted by their original authors. However, self-deleted questions cannot be edited by their authors unless undeleted first. Self-deleted answers can be edited by the author without undeleting. Normally, if your deleted post is not self-deleted, you can't undelete it yourself (though you may be able to vote to undelete). An exception: if an answer is deleted from Low Quality review queue by "Recommend Deletion" votes, without three trusted users voting to delete, then it can be undeleted by the author.
This implies that if a post is not self-deleted, then it can be edited by its author. This allows a post that is deleted by others to be improved so that it can be undeleted. However, if the post was deleted more than 60 days ago, you must have a link to the post or have it marked as a favorite to be able to find it.
Yes, provided that
you have the appropriate privilege.
you have access to the post.
the post was only deleted and not also locked (and is not a self-deleted question, see other answer).
If it is about your own post mainly the last point is relevant.
As locking is mainly reserved for offensive or spam content and you could just undo a self-deletion, you should mostly be fine (unless you lost trace of the post).