Don't worry if you archived something by accident - you can always restore it completely intact later on.
You can access the archive in your profile settings. Click on your profile icon within the navigation panel and then select the profile icon again. In the bottom section, you will now find "Archive". Click on it to open it. The archive is divided into Workkspaces, Collections and Items. Use the search bar to search specifically for archived content.
Hover over an item until the "..." icon appears. Click it to restore or permanently delete the item.
You can also click on the item and open it in the detail view to check if it is the right one. From here, you can then click on the three dots in the upper right corner and select "Restore Item" or choose to delete it permanently.
The Collection Archive
The collection archive is located within the collection settings. Click on the collection name in an open collection to get there. Then select "Archive" on the left side of the window. In the collection archive, you will find a list of all archived items. You can use filters to search for specific items and restore or delete them permanently.
Either restore the items collectively or hover over the respective item and select the three dots to restore or delete individual items.