After Moriarty murders Dr Watson, Sherlock is left in turmoil, unable to forgive himself for his failure to rescue John.
Taking to excessive smoking and becoming a hermit in his grief, it seems that Sherlock will never leave Baker street again. But this all changes when The Doctor arrives on his door step - proving to be his greatest case ever.
Only four people know of the true state Sherlock Holmes was in just before he met John Watson. The darkness, the drugs, the voices, her. You could argue that her death triggered it. Now that John has left Sherlock to live with his wife it is inevitable that Sherlock will go back to how he was. Until somebody knocks on his door. He wasn't the only one to fake his death. (Set in between TSoT and HLV)