If it went from 25' deep to 5' deep you have an enormous amount of muck to deal with, that being said, imo, raising the dam height will be a like a small and somewhat ineffective bandage for a bigger problem and even if it somewhat worked it wont be a beautiful useful body of water, cutting draining and cleaning it out will result in a much better product, although a little expensive, probably comparable to the cost of building one from scratch, but you would have basically a brand new pond. and you might think about puttin a silt catching pond right above it to avoid as much muck in the new pond, and could also be used as a forage pond. Good Luck!

All the really good ideas I've ever had came to me while I was milking a cow.