Icicle

Braxton Thorpe has discovered a threat to the entire Solar System, but he has a problem: he's dead.
Frozen at death, he awakens to find himself uploaded into an electronic matrix. Exploring beyond the matrix and the larger GlobalNet, he discovers the Oort, a distributed electronic entity older than humanity, with an unnerving secret: aliens wiped out nearly all life on Earth once, and are coming back to do it again.
The mathematical entity that is Thorpe has to find a way to convince humans of the threat, and in time to do something about it. But how, and what?
If you've read Niven's "A World Out of Time" or Taylor's "We Are Legion," the opening of "Icicle" will only "seem" familiar. Buckle up for a wild ride; you ain't seen nothing.