I went to add the last 100 cards I had added to my inventory to a deck and accidentally hit the delete selected instead of add to deck not only killing an hour work but also removing cards that where multiples that had been added from previous boxes/folders.

Ps anyone know how to undo this as this has created a large missing chunk of already inventoried cards

Submit a support ticket, and I don't know if it matters, but I'd avoid making any changes to my inventory until the support ticket is completed.

Sorry to hear! That sucks. Another recommendation for the future is to always keep a backup of your inventory exported to CSV (you can find Export under Tools). Then if something like this ever happens again, you can just reimport your entire inventory from that CSV. :)

I submitted a ticket. I agree with your advice. I've just been mass inputting the last few days. Never even thought about it. Lessons learned.