Doctor Who season 12 destroyed the Time Lords, but there are several established methods set for their return. In a shocking twist, Doctor Who season 12 revealed that the Master (now played by Sacha Dhawan) destroyed Gallifrey after finding out the secret of the Timeless Child - a plot thread teased back in one of Jodie Whittaker's earliest episodes. The Master reveals that the truth he discovered about Gallifrey was so damning he could only destroy his homeland in response. The Doctor checked Gallifrey for herself, finding the planet a desolate wasteland, but remains none the wiser as to what the Timeless Child is.
Just because Gallifrey is in ruins, however, that doesn't spell the end for the Time Lords on Doctor Who. The aloof race has a habit of returning from the grave, and season 12's revelation is unlikely to end any differently. Come the season finale, the Doctor will surely be looking to investigate the Timeless Child herself, and this will most likely necessitate another visit to her home planet at a time period when there were actually people there to talk to. Fortunately, Doctor Who already has several ways to facilitate the return of the Time Lords.
The first potential method is alluded to in Doctor Who's most recent episode (via RadioTimes) and involves the teleportation cube Graham puts together to whisk the gang to their Tranquility Spa vacation. Although this tech ended up being fairly innocuous, the item reminded many fans of a Gallifreyan hypercube. First appearing in the Second Doctor's final story in 1969, hypercubes are essentially USB devices for Time Lords, able to record words and thoughts and be safely sent through the universe to other members of the species. Hypercubes reappeared in modern Doctor Who during "The Doctor's Wife," where they were treated almost as monuments to dead Time Lords. Although Graham's cube might not have been Gallifreyan in origin, its appearance might provide a visual Easter egg as to how the Doctor will be able to communicate, or even resurrect, the Time Lords later in season 12.
Cuboid communications aside, there are plenty of other established ways the Time Lords could make a comeback, assuming the Master did actually succeed in wiping them out. One of Gallifrey's greatest inventions in their Matrix (not that one), which acts as a database of all the Time Lords' collective knowledge about the past, present and future of the universe and is used to predict key events. Far more than just a fancy Excel spreadsheet, the Matrix is a universe unto itself and Time Lords have the ability to mentally transfer into it. As a means of last resort when the Master set his sights on destroying Gallifrey, fleeing into the Matrix could've been a possibility, and would give the Thirteenth Doctor a potential means of reviving her people.
Of course, the Time Lords have faced certain doom before during the Time War, and Rassilon's solution was to break through the time-lock surrounding the event and try to escape in the Tenth Doctor's final outing, "The End of Time." Given the powers available to them, certain Time Lord leadership might decide to try the same trick again, escaping the Master's destruction by busting out of their bubble universe (now there's clearly an easy way in and out) and relocating elsewhere, leaving Gallifrey behind to burn. This would be an interesting dynamic for Doctor Who, as it allows for Time Lords to potentially appear anywhere in the series, rather than being confined solely to their home planet.
A final option for Time Lord survival might be to use a Genesis Ark. Although originally designed as a prison ship for Daleks, the ark helped protect hundreds of the Time Lords' enemies as they escaped the Time War and lives up to its title. While it might not be a pleasant experience, using that same technology could help protect the Time Lords from an attack that would otherwise wipe them out but, of course, the ark can probably only be opened by a time traveler from the outside. Cue the Doctor?
Doctor Who continues with "Nikola Tesla's Night Of Terror" January 19th on BBC and BBC America.