Dejé de existir literally means "I ceased to exist". It's a dramatic and poetic phrase that perfectly captures an overwhelming, all-consuming feeling.
In the song, Luis Fonsi uses this hyperbole to describe the moment he first saw someone. He sings, "Cuando te vi / Todo se fue en un momento y dejé de existir" (When I saw you / Everything went away in a moment and I ceased to exist), expressing that he was so captivated that everything else, including his own sense of self, completely faded away.
Luis Fonsi wastes no time playing cool here. Right from the start, he admits that while you might not want another speech about romance, he cannot keep it to himself anymore: "pero es que ya no me aguanto las ganas" (but I just can't hold back the desire). In Spanish, no aguantarse las ganas is a natural everyday idiom for being completely unable to stop yourself from doing something. His pitch is direct, almost breathless: tell me you are single ("dime que estás sola"), and I promise you will wake up to a kiss every single morning ("siempre con un beso vas a despertar"). It is bold and completely over the top, but his charm sells it.
He frames this encounter like an absolute reset on his life. Seeing her literally made the world stop: "dejé de existir, como perdido en el tiempo" (I ceased to exist, like lost in time). He even claims that before her, he had lost his ability to sing ("me volvió la voz para cantar"), and that he never even understood how to fall in love until she walked in ("yo nunca supe enamorarme y me pasó contigo"). It is pure, unapologetic pop romanticism where finding the right person instantly turns night into dawn and solves every missing piece in life.