At the moment this is just a rather disorganised list of all the things I can think of that can (and have) gone wrong. So if anyone feels like trying to tidy up this page, please do so!
Unfortunately, this problem could be caused by any number of issues:
If you can’t find anything obviously wrong with your setup, and rebooting doesn’t make the issue go away, then you will most likely need to connect a serial cable from the board to another PC in order to determine why the board isn’t booting. If you’re unsure how to do this, just ask on the forums!