What would the plural be, nowych nagrod?
yes, nowych nagród
Co się stało nad starej nagrody..?
I believe that you meant "What happened to the old prize", which would be "Co się stało ze starą nagrodą?"
"Stara nagroda" is Instrumental, and "z" turns into "ze" because of pronunciation reasons.
Does this refer to a prize as like a gold medal or to the price of an item in the supermarket?
Gold medal.