It could have been a fungal infection. If the frogs were housed together, I'd take the survivor in to the vet, as this is highly contagious.

I had one with spots on the face that I took to the vet and started treating, but he didn't tell me it was so contagious and I did not separate them. The other frog never had any spots, but died suddenly - the vet said he probably had the fungus internally.