Does your flash work? I ask because the power button and flash are on the same flex.
Normally, changing a screen should not have any interaction with the power button connector or flex and none of the circuitry is anywhere accessible for just a normal screen replacement.
Did you remove the logic board or anything other that what is saidin this guide?
Think back about what you did and try to remember if your screwdriver slipped or anything else that may have damaged the flex or button.
You may want to look at the power button from inside the phone with a magnifying glass or jeweler's loupe to see if the mechanism is indeed touching the actual actuator on the flex. If everything looks okay try to disconnect the battery one last time. A hard reset would be nice but without a working power button that is difficult. You could try to reset your device via iTunes.
Otherwsie, I would try a new flex and follow these instructions.