I deleted a card and it was lost for good. However by going to the Board's Activity log, I was able to see all of the transactions to the card that I lost going back quite a while. I was able to get all of the info I had lost, and recreate the card. It's not quite as good as recovering what was deleted, but at least it's better than a complete loss.
Go to the board of the card you want recovered. From the menu select Activity. (I found this easier to do on my cell phone using the '...' menu. I'm sure you can do it on a laptop/PC, it just wasn't as clear to me.) Once you've selected Activity it will show you each of the transactions that have occurred on that Board, including all of the line items from a card that was deleted. You can create a new card and manually re-enter the missing data. This is a laborious task depending on how much info was on the card. For me the data was so critical that it was worth the effort to manually recreate it.
I was so hopeful with this answer as I found the Boards activity log and scrolled down but it only shows board activity. It doesn't show comments and such. I need to vent. I had a board with all and only most important user names and passwords. It was my most valuable board and I deleted it because an attachment became the cover photo. I was trying to delete the screenshot and deleted the whole board. There was no,are you sure warning. I'm very upset!