The central conflict of The Time of the Doctor revolves around a stalemate between the Doctor and his allies, his enemies, and the Time Lords. The Time Lords, who are
trapped in an alternate dimension, after the events of The Day of the Doctor
are sending out a signal throughout time and space, which only the Doctor can answer. If he provides them the answer, they will know it is safe to
re-enter the universe.
All of the Doctor's enemies have gathered at Trenzalore to prevent the Doctor from answering the question. The Doctor himself also refuses to answer the question, because doing so would bring the Time Lords back right smack in the middle of the collection of the Doctor's enemies, and thus the Last Great Time War would begin again. But at the same time, he refuses to just leave, as doing so would open the town of Christmas up to attack from those enemies seeking to ensure the question is never answered.
So there is a stalemate. The enemies won't leave, because there's a chance the Doctor will answer the question. The Doctor won't leave because his enemies will attack the town.
At the end of the episode, however,
the Timelords grant the Doctor a new regeneration cycle and he uses that extra energy to obliterate the masses of enemies gathered at Trenzalore.
From there, the stalemate appears to be over. The Doctor does not seem to have a reason to remain silent. He could answer the question and bring the Time Lords back. But he doesn't. And yet several episodes of the following season involve the Doctor attempting to "find" the Time Lords. Why? He already knows where they are and how to get to them. Just go back to Trenzalore and answer the question, and they'll come through. Did something happen in that climatic end of the stalemate that rendered the whole thing pointless, removing the ability for the Time Lords to return after all?