In more detail....

Pretty much a Shinigami killed Central 46, then traitors began attacking.
Arrancars showed up and attacked the Seireitei doing lots of damage.
Head Captain turned out to be a traitor as well, working with the other Shinigami and Arrancars.

Some guy showed up who is now the new Head Captain (Juki), and with his magical mystic voodoo powers fixed up the whole Seireitei.
Three month time-skip since then, people going about their days but training also.