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I accidentally hit c with a card selected and archived the card. I tried to undo the archive but couldn't find the option. I tried moving the card back to the list but that didn't seem to work.
In the top right select Show menu
Then click More
Then click Archived Items
You then click the Send to Board button on the item and it will be reinstated to your board.
Send to Board
You can also visit your profile and see the audit trail of what you've done to each card, which includes a link to the archived card. Then you can "move" the card back to the board.
Click < Show Sidebar then Menu then Archive to see your archived items. Once there you can add them back to your board.
< Show Sidebar
There is an option in the card menu, counter-intuitively named "send to board"
There is a discussion for android app: this option seems to be named something like "unarchive", which is inconsistent but easier to understand. Stay tuned for modifications.
There appears to no longer be an Archived Items menu item on the board's menu. I was only able to find an archived card by searching for it.
Required, but never shown