Quedarse+participio is one of the most common ways of expressing a sudden state change and resulting state, especially for surprise or shock. As well as the participle, there are several expressions that tend to go with quedarse, including de piedra, de una pieza, muerto and a cuadros. [Remember: quedarse alguien+participio can usually be paraphrased with dejar a alguien+participio]
-Se quedaron muy sorprendidos con la casa— They were very surprised by the house
-Se quedó alucinado cuando lo vio — He was gobsmacked when he saw it [Véase alucinar]
-Me quedé muerto al oír la noticia — I was stunned/shocked when I heard the news
→Me quedé con la boca abierta
-Cuando me dijo que me había puesto los cuernos me quedé de piedra — When he told me that he had cheated on me, I was stunned
-Se ha quedado de pasta de boniato — He's stunned
-Me quedé a cuadros — I was flabbergasted/astonished
→Me dejó a cuadros [Con el verbo 'dejar', el sujeto es la cosa que causa dicha impresión - leer más]
-Al decirme que había matado a alguien, me quedé helada — When he told me he had killed someone, I was stunned/terrified
→Su confesión me dejó helada [≈Sorprendida o asustada]
