Spice Jet Airlines — damage to musical instrument - harmonium

Our group of 9 members were travelling from bengaluru to mumbai on 4th june 2018 by the 8.10 pm spice airlines from bengaluru. Airline pnr number was cy9ukg and the tickets numbers were cy9ukg/1/1 to cy9ukg9/1. We requested for permission to carry our scale harmonium instrument as hand luggage due to the fragile nature of the instrument. However, the counter officials refused and we were per force book as a cabin luggage. After reaching home, on the next day when it the box was opened it was found that the entire keys of the harmonium have become loose and it was not possible to play the instrument. The same has been given to the mechanic and the damage is being assessed. The harmonium cost is about rs.25000/-. Please help us to get suitable compensation from the airlines.