Marchitó comes from the verb marchitar, meaning 'to wither' or 'to fade'. It's a vivid and somewhat melancholic word often used for plants, but here it's applied to emotions.
In the song, the lyric "Si nuestra flor se marchitó" (If our flower withered) uses this word metaphorically to ask if their love, symbolized by a flower, has died or faded away. This poetic imagery makes it a memorable and emotionally resonant word.
This song is a straight-up plea from someone who's been left completely confused and heartbroken. The singer wakes up every day looking for the person who's gone, but all he finds are "Solo recuerdos" [Only memories]. He's stuck, unable to move on because he doesn't understand what went wrong. He directly asks his ex for an answer: "Dime, mi amor / Si nuestra flor se marchitó" [Tell me, my love / If our flower withered]. He needs to know if it's really over so he can stop looking for the love they once had.
The pain comes from a broken promise. He remembers them swearing, "De aquí no me voy, juraste, mi amor" [I'm not leaving here, you swore, my love], and now he's left wondering what happened. He insists, "No te di razón para que te vayas de mí" [I didn't give you a reason to leave me], which makes the whole situation even more frustrating for him. He's just searching for closure from someone who left the door "tightly closed" behind them.