I used to be this guy sittin’ with an open heart
Now my computer screen’s the only place I feel a spark
He was scarred by what happened with his old girl, so he feels he can’t truly connect with other girls because he’s paranoid it’ll happen again. The computer part is referring to how his work makes him feel whole and complete; how his work won’t let him down. He can trust in his work, as it’ll always be a safe and honest passion. As far as he’s concerned, that’s the only real thing left in his life since his rise to prominence.
Also he gets a spark from pornography on the computer and not from “real” girls anymore and it also could be a reference to the literal static electricity you can feel by proximity to the screen of a CRT monitor contrasted to the figurative “spark” of being in love, which he no longer feels.
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