A dark screen after a build is disappointing, and it is usually a solder joint that has not quite taken rather than a faulty board, so in most cases you can put it right with the iron you already have.
Look for joints that are bridged across to a neighbour, or that have not flowed properly. Reflow those and power the phone up again.
A good joint:
has a volcano-like shape
covers the whole golden pad with solder
leaves the black parts of the board clean, with no stray solder on them
The photographs below show examples of good and bad joints.
If the screen is still dark after that, email us at [email protected] with a photo of the board and we will go through it with you.