Hi Maxdog
There could be a number of reasons for the unfortunate number of dead frogs you have found: -
1) It sounds as though your pond froze over quickly, trapping some frogs in the ice. The ice would freeze their internal tissues, slowing down and ultimately stopping vital biological functions, enough to kill the frogs. Once the ice thawed, the dead frogs would just be left floating in the water.
2) When ice covers a pond, it stops oxygen and other gases entering and exiting the water. Natural decomposition of debris at the bottom of the pond uses oxygen in the water and gives off carbon dioxide (CO2). As it can not escape from the water because of the ice layer, the CO2 levels in the water can get too high and as a result, aquatic life can start to die off. Amphibians are able to "breath" enough oxygen through their skin to enable them to lie dormant in the water; if the oxygen is depleted and CO2 levels build up, this would also kill frogs in the water, due to oxygen starvation.
3) Predation. Something may have turned up in the night and had a "field day" picking off your frogs who were unable to escape to the deep water due to the ice. However, if the bodies were unscathed and had no obvious marks or wounds, this is unlikely.
4) Ranavirus (Redleg). This is a virus hitting frog populations. It causes redness, blistering and ulceration of the limbs - often leading to the loss of the unfortunate frog's hands and feet. This is a long and lingering affliction, which eventually kills any infected frogs. It does not tend to cause the "sudden" deaths you seem to have experienced in your pond. Unless the dead frog's have any of the visible symptoms, it is unlikely that this was the cause.
I would advise against moving the spawn, to prevent risk of cross contamination from one pond to another (just in case); and let nature take its course. I would also advise against such drastic action as draining and cleaning the pond out. This would have a devastating effect on the other things (invertebrates etc) habiting the pond, not to mention its balance and chemistry - it would take months, if not years for the pond to get back to normal again. It sounds to me that other than your mysterious dead frogs, your pond is healthy. Distressing though these sudden deaths may seem (I have had similar in my pond too); amphibian mortality can be quite high. They are fairly vulnerable, highly predated and are a vital component of the food chain (one reason for the large amounts of spawn produced), but the frog populations generally do and will recover.
It is worth floating a ball in the pond during colder months. When ice forms, the ball can be removed allowing gases to enter and exit the water. Alternatively, if ice has formed and you want to make a hole in it, place a bowl of warm water on it until a hole forms. Don't be tempted to break the ice by hitting it. This can not only damage your pond liner, but can also kill creatures living in the water underneath the ice. Whatever - do be careful, and don't end up falling in yourself (or kids who may be helping).
Also, it is worth pointing out that some frogs pulled up from the bottom of the pond might just still be lying dormant (hibernating). Unless they are very obviously corpses (displaying signs of decomposition etc), just be wary of disposing of what might actually still be living frogs, deep in their winter slumber. If in doubt, leave them be. Dead ones will ultimately float to the surface anyway.
I hope your frog populations soon get back to normal and I hope this (rather large!) response helps.
Jono
Very strange that you have a fair number dead, I haven't seen any other posts relating to frogs dying of the cold this season as yet 
I have no idea but am very interested in the replies . 
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