Ygdrassil

Damaged by Hel - she severed the link between Heavens and Hells, effectively destroying the 'trunk' segment, most commonly associated with the Prime. Ultimately, doing as much damage to her own realm as to anywhere else.

As a result - the Astral plane no longer exists as a transport layer because it was synonymous with Ygdrassil. The 'upper planes' are still inter-connected but not linked to the Prime or the 'lower planes'