Whether or not an item or part can be recovered if it was mistakenly deleted depends on many factors. This article describes various scenarios. 

Deletion of Version 1 of an Item

If the item deleted is Version 1, then the only way to retrieve it would be from a backup of the database (for example, in a test environment) assuming the item is in the backup. You would then have the content to be able to recreate the item again in production. 

Deletion of Subsequent Versions of an Item

If there are earlier versions of the deleted item or part, then you could look to an earlier version to recreate, at least in part, the deleted item.  

Deletion of a Part from a Structure

If a part has accidentally been removed from a structure (by right-clicking on a part and selecting "Remove part(s)"), you can simply re-add it (since the defining item still exists in the database) by right-clicking on the parent item and using Add and selecting the item again from the item type list, thus recreating the deleted part.