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
You then click the
Send to Board button on the item and it will be reinstated to your 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.
< Show Sidebar then
Archive to see your archived items. Once there you can add them back to your board.