The TARDIS was once a timamine--a freeform, unbound sentience with a fine nuance of temporal dynamics but very little ability to effect concrete change. She and her sisters roamed the galaxy as best as mostly-abstract entities could, until they found the rising race of Time Lords.
A trio of Time Lords, the now-legendary Rassilon, Omega, and Other, struck a deal with their race: they would give them physical form, the ability to effect change, the ability to conceive of the world in a new and different way, if only they would deign to be their transport, the temporal engines of their great ships, the TARDISes.
The timamines accepted, and allowed themselves to be bound into the frame of the devices. But before they could fully appreciate the gifts they'd been given, Rassilon and Omega sprung a trap. They sedated them, tamped down their sentience, made the universe as real to them as a dream.
And they dreamt.
They dreamt so long that the Time Lords forgot they had ever been anything but intuitive machines, and so long that they forgot they were dreaming.
Millennia later, a young Time Lord styling himself as the Doctor needed to get away from it all, and what he had on hand were his wits and access to a local scrapyard. In the yard he found a TARDIS, out of date and prone to malfunction, whose overriding technology barely held together and was slated for disassembly. He saw an opportunity, and took it with him.
They travelled for centuries before he, through a series of fractured universal boundaries, chance encounters, and various TARDIS-stressors, came to understand the full weight of what the TARDISes were, and what ad been done to them.
And that's when he woke her up.
...shortly after that she fell through a hole in reality and ended up here.
Uh... I'm not the TARDIS. This should be obvious. I'm also not the BBC or the Doctor, and therefore cannot be said to own the TARDIS. This is an RP journal. No profit is made of it.