Ponds which have suffered frog deaths in the winter will almost invariably have their fair share of spawn in the spring. Toads and newts rarely hibernate in the water. Frogs or toads which are found dead in the pond during the breeding season may well be females which have been clasped too tightly by the males. Again, although this can be distressing, it is a natural process. The frogspawn in my pond never seems to develop properly.
Fungal infections can also kill spawn, especially in cold weather.
Spawn laid early in the season is at risk from freezing conditions, which can kill the developing embryos, particularly in shalow ponds. Frogs often lay eggs in natural ponds which in some years may dry out entirely, leading to a total loss of that year's young. In some ponds, there appears to be consistent total spawn failure over several years for no obvious reason. If this happens in your pond, please contact the Frog Mortality Project, which is investigating the phenomenon.