Don't worry if you archived something by accident - you can always restore it completely intact later on.
Search and restore
Search for the item using the global search, search for the name of the item in the "Archive" tab, then click on the vertical "…" symbol next to the name and click "Restore Item".
You can also click on the item to open it in details view to check that it's the right one. From the details view, you can then click the "..." icon and choose "Restore Item".
The item will be returned to the original collection with all details and labels intact.
Restore from collection activities
Don't worry if you can't remember what your item was called, or can't find it in search. You can find archived items by looking through the collection activities.
Click on the heartbeat icon in the top right hand corner to open the collection activities.
Under the "Activities" tab, you'll be able to see an entry about the archived item (shown as a struck-through item name). Once you've found it, just click on the item name and follow the steps above to restore it.