First of all I am so very sorry for your devestating loss. Please accept my sincere and deep sympathies for your agony. I hope you don't blame yourself -- no matter how carefully we "puppy proof" our homes they can always find something to get into. I can assure you that eventually the pain you are feeling right now will become a dull ache and then one day you will remember your precious baby with a smile. At that moment in time, your healing will begin.
I, too, thought I had puppy proofed my home a couple of years ago when I had two litters of pups (9 total) running all over my house. It seems that there was ONE electrical cord -- a new one that went to an aquarium filter that was unprotected, but was "hidden" behind a large piece of furniture. To make a long story short, one of the pups found the cord, and apparently a second pup was touching the biter cause they both were knocked out. We were very lucky, both pups recovered with no after-affects. But it was a stark reminder to me not to take anything for granted.
Now I do puppy safety checks by crawling on my belly with my nose to the floor.