Female Fallaen Angel

Start with what her alignment is now. The fact that you mention the Demon Lord template implies that you are thinking of her as a Demon (and now an exemplar of CE). Demon "Lords" are very powerful indeed, and are minor Powers in the broader scheme of things (sorry if I am using too much jargon here). She will only be a Lord per se, if she is command of an entire Abyssal Layer. If she just a really badass Demon then she isn't a Lord. In terms of deities and reasons for falling - again that is totally up to you as DM to decide and explore. There is quite a lot of discussion about this on the Planescape website, so I tend to just browse through the relevant threads here for ideas.
In canon, off the top of my head, Pazuzu (in some accounts) was a Celestial who fell before there was even a defined concept of good and evil to become an Obyrith. In my campaign he was once one of a vast host of lawful Angels that maintained the basic function of the universe and the planes in its early days. I consider him to have been broken by exposure to extra-dimensional horrors such as Tharizdun and the Far Realm Powers when the Abyss itself was still forming.
Another Demonic fallen is Adimarchus the Prince of Madness, who was a Celestial Archon who was tricked and betrayed repeatedly by the machinations of Graz'zt until he eventually went insane.
The Obyrith Lord Sertrous also managed to corrupt and break an Archon to fall simply through cunning philosophy and rhetoric.

Baatezu (LE) fallen are seemingly more common - as they are basically just Lawful angels who have started to commit morally questionable actions in the name of their cause - and justify this through fanatical devotion to their orders. They are like classic anti-paladins, whose zeal has eroded their virtue. In canon, erinyes devils are the most common fallen Baatezu, so if I was going to make a LE female fallen angel I might build her around an erinyes to begin with. More powerful fallen Baatezu include Asmodeus and Baalzebul.

NE Fallen are a bit rarer in canon. They tend to have succumbed to despair, apathy or spite. Off the top of my head there is an Altroloth Fallen called Xenghara - but I don't know if he is statted out in 3.5 ed. at all. There is definitely a yugoloth/baatezu Fallen in Champions of Ruin though. Green Ronin's book on Gehenna totally rebuilds this Plane, so doesn't fit normally into Planescape without some rearranging - but there is a statted out fallen in there too called Ahrimanes.

Once you have the before and after exemplar Alignments you can work out the why and the who. Her original alignment is important too because then you can work out what kind of Celestial she was to begin with.

Not all fallen gain power after the fall, Zalatian the trumpet archon in the Blood War adventures, falls so low he has to beg in the Hive.