The story is that the Alter Rebbe was once reading from Pekudei 38:28, which mentions the sum of 1775 talents collected.
When the Alter Rebbe read it as “5775” his Gabbai corrected him. The Alter Rebbe then said again “5775”, to which the Gabbai once again corrected him. This repeated itself 4 times. Thereafter the Alter Rebbe went into deep meditation. When he quit his meditation he opened his eyes and whispered to the Gabbai that “5775” had something to do with the coming of Moshiach.
Rabbi Groner verifies the authenticity of the Rebbe previously revealing this story to at least one of his secretaries.
It is believed this story's source comes from an as yet unknown "Polish" sefer.
(And, as a blogger Rafi points out, here, 5775 = תשועה
and "every time the word תשועה is mentioned in Tanach, it always has to do with David. No exceptions.")
That's David as in "משיח בן דוד".
Let's hope the Alter Rebbe's prophetic story is confirmed.
This link, however, says the 5775 issue best!