Something that's ruined is spoiled, wrecked, or destroyed. A ruined party can result from your obnoxious brother annoying all the friends you invited.
Ruined means destroyed or useless. When you visit the site of a long uninhabited city, the broken down structures you see are ruins — time and neglect has resulted in ruined buildings. Both words come from the Latin ruere, "to fall." Opening the oven door too soon can yield a ruined soufflé — it could literally fall, or collapse into a squishy mess. Your reputation as a cook might also be described as ruined after such a disaster.