A time lock was a mechanism whereby an event or series of events was rendered unreachable by time travel. It was thought impossible to pass through a time lock, but this was proven false.
The Doctor's TARDIS possessed a time lock. (GAME: TARDIS) When the TARDIS was suspended in space by the Great Intelligence, the Second Doctor wondered if this time lock had slipped. (TV: The Web of Fear)
By the Doctor
When the Doctor used the Moment at the end of Last Great Time War, the entire war was time locked, (COMIC: Don't Step on the Grass) which prevented entry into the Time War through the time vortex. This meant that the Time War's events were not erased from history; they could still be referenced by those who knew of the destruction of the Daleks and the Time Lords. The Doctor, however, would not be able to reach the Time War through time travel. Any entry into the events of the Time War would be a change in the timeline, made impossible by the time lock. [additional sources needed]
By Time Lords
In ancient times, the Time Lords used time locks to hide their secrets and misdeeds. When Skagra used The Worshipful and Ancient Law of Gallifrey to access Shada, Romana hypothesised that they were passing through a time lock. (PROSE: Shada)
Davros suggested once that Emperors and Time Lords had attempted to breach a time lock, but failed. Dalek Caan overcame the lock on the Time War after making an emergency temporal shift and travelled back to the Gates of Elysium to save Davros. The process flooded Caan's mind with total knowledge of the past, present and future, driving him insane. (TV: The Stolen Earth) The only other things capable of breaching the time lock were those that were already in place, like the signal that had been transmitted back through time into the Master's mind and, on at least one occasion, very small objects, such as the White-Point Star diamond sent by Rassilon to follow the signal in the Master's head. (TV: The End of Time)
Another sort of time lock, created by Torchwood 3's resident genius Toshiko Sato, was designed to freeze the Torchwood 3 Hub in a bubble in time. It was at the same time impenetrable but inescapable. The time lock was either shut down or damaged when a Dalek trapped at the edge of the bubble was blown up by the Meta-Crisis Tenth Doctor. (TV: Journey's End)