Sirius is not dead.
The veil transported him back to the tropics and he is living on an island, where he is busy raising Hippogriffs and training giant birds to send letters.
Unfortunately, the veil is coated with a unique memory charm that can’t be explained (thus it’s placement in the Department of Mysteries) and so Sirius has no recollection of Harry or the Order which is why we haven’t heard from him.
He died before he fell through the veil, sweetie. He was hit with the killing curse.
With your record... I don't know if you're joking or being sirius (I had to, xD). Killing curses are green. The spell that hit Sirius was red. (Although we never knew what the curse did). Sirius falls through the Veil (probably from the impact of the curse) and with the DH Forbidden Forest scene it's confirmed that the "Death Chamber" was indeed a chamber of death...
(With that, J.K killed my headcanon of Sirius being in a Limbo kinda state between books 6 and 7).
