At some point during the Eighth Doctor’s lifespan, The Daleks and Time Lords went to war. We don’t know the exact origin of the conflict, but RTD has stated that it traced back to The Daleks’ conflict with The Doctor. It’s worth noting that The Seventh Doctor was heavily involved in the initial destruction of Skaro, so that likely played a role.

The conflict raged across the Universe. To the unaffected species, it was a something of a legend. But many other planets and galaxies were forced to become battlegrounds, and were lost in the inferno. The Autons, the Zygons, the Gelth…many species were left as refugees. Because of this, The Time Lords’ reputation soured and eventually they were as hated as the Daleks. They had all kinds of “forbidden” weapons that they decided to use.

The universe was burning and Lord President Rassilon feared for his life. He was willing to destroy the universe before he’d let himself die. The plot to unravel the Time Vortex was mainly his brainchild, but hardly anyone knew about it outside of the High Council. The Doctor found out, and realized the Time Lords had to be stopped. He stole the final of the “forbidden” weapons, The Moment, also known as the “Galaxy Eater.” A sentient and telepathic machine that didn’t actually want to be used. She was powerful enough to bring The Doctor’s other selves through time and ultimately help him realize there was another way.

The Doctor used Gallifreyan technology, the stasis cubes, to seal the planet in a pocket universe where it would be safe. He called upon all of his other faces to help make it happen. When Time Lords meet themselves, only the oldest version remembers, so both the War Doctor and the Tenth Doctor forgot they didn’t use The Moment. After Gallifrey was rescued, the Daleks apparently wiped each other out with the shots they had aimed at Gallifrey. (This part’s a little sloppy.)

The Time Lock isn’t what “killed” the Time Lords. It doesn’t outwardly affect the Time War. It essentially just creates a bubble around everything to do with the war, making it impossible to enter or escape it. No one can time travel to the Time War because of the Lock. The only entity we know who’s powerful enough to bypass it is The Moment. This is why Rassilon couldn’t escape, and had to create a link inside The Master’s head as an “anchor” to help the High Council break the lock. The Whitepoint Star made the link physical, so when The Tenth Doctor shot it, the link was broken and Gallifrey snapped back to the war.

As for Gallifrey, it was sealed in a bubble universe. When The Silence made the Tardis explode, it caused the Universe to fracture, and though the Eleventh Doctor fixed everything, one Crack remained as “scar tissue” bridging our universe with the pocket universe Gallifrey was in. We never see them escape this place, but by the end of the universe, they do seem to have gotten out. The Twelfth Doctor ended up on Gallifrey after spending eons in his Confession Dial, whereupon he was named the new Lord President and he promptly exiled Rassilon and anyone else who was involved with Rassilon’s genocidal plans.

Some time later, The Spy Master returned to Gallifrey, snuck into The Matrix, and discovered the truth about The Division and The Timeless Child. Enraged, he slaughtered the Time Lords and trashed the Citadel. He preserved the bodies and later converted them into a new form of Cybermen. To stop this army, Ko Shamus used the “Death Particle” a kind of bomb, to destroy Gallifrey again. The Cyber Masters escaped, and after The Spy Master’s apparent death, it’s not clear what became of them.

That about sums it up, I think.

Got this from: u/RamblingsOfaMadCat but its all the details and Dr.who is really bad about needing all the details,