sorry to hear that he/she died - As H-Man has said - you MUST have a Ammonia test kit - you will also need a pH test kit as well - but the first Ammonia is the most important - If you have one - what was your reading?
Also make sure there is plenty of oxygen in the tank - if your not sure - ask.
For a healthy discus to die - there is something wrong - Most discus will chose to eat rather then starve to death - they aren't dumb fish - this is why it is vital to test your water. When the water is in good condition - you will know because your Discus will show you - their fins will be completely flared - their color will be bright and beautiful (for colored fish that is) and they will never shy away from food - unless their stuffed - in that case their being over fed.
I would also drop your pH from 7.6 - Discus prefer more acidic water - Though i have never 'healthily' kept them in water above a pH of 7 - so i will need someone to confirm it is ok to keep them in this pH range please?