Sounds like you killed it to me. Sorry. Sometimes it is possible to damage the handset when performing a repair. Been there, done that.
I have changed out 2 iPhone 4S power buttons in the past week or so and both would not boot past the apple logo after. What looks like DOS prompt and code starts running and possible a security error. I change broken screens all the time and have never seen this happen until I changed the power buttons. Is this a new security feature? Is there a work around?

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