I cancelled a purchase order by mistake. Can I bring it back?

No. Once an order is received or cancelled it is closed, and a closed order stays closed so that a delivery can never land on the shelf twice. Create a new one instead. Open Menu → Purchase orders, choose the same supplier and open Fill to par. Because the cancelled order no longer counts as stock on its way, every ingredient it covered is listed again with the quantity you need. Press Create the order and you are back where you were in under a minute. The cancelled order stays in your list so the story is still readable.