Descongelará comes from the verb descongelar, meaning "to thaw" or "to unfreeze". It's an evocative word that you don't often hear in song lyrics, making it stand out.
In the context of the song, it's used metaphorically to describe the emotional state of a relationship that has grown cold and distant. The repeated question "¿Quién nos descongelará?" (Who will unfreeze us?) powerfully conveys a longing for warmth and connection to revive a dying love.
Dani Martín and Camilo build the whole song around one small image: a paper airplane that got rained on and won't fly anymore. “Avioncito de papel” (Little paper airplane), “Que se mojó, no vuela más” (That got wet, doesn't fly anymore). It isn't really about paper planes. It's a relationship that ran out of momentum, described through a blackout: “Que no hay electricidad” (That there's no electricity), “Se apagaron las farolas” (The streetlights went out).
By the second half the plane turns into a paper boat that sinks instead of flying, “Ay, barquito de papel” (Oh, little paper boat), “Que se mojó y se nos hundió” (That got wet and sank on us), and both singers keep asking the same question, “¿Quién nos descongelará?” (Who will thaw us?). Nothing in the song is loud. That's the point: “Ya no queda ni un pequeño movimiento” (There's not even a small movement left). The relationship didn't end in an argument. It just stopped moving.