system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What would happen if the emperor of mankind miraculously woke up, were full strength and stepped off the golden throne?
gpt
Unless he imposes his authority absolutely pretty much as soon as he wakes up, the Imperium will fracture. One side would follow the Emperor without question. Another side would denounce the Emperor as a fake and refuse Imperial authority and another side would take advantage of both sides.

The Ecclesiarchy would either implode internally, or be completely stripped and rebuilt into some kind of secular missionary organisation.

The Inquisition would splinter spectacularily, with some inquistors going rogue to try to maintain their level of influence, their ex-fellows hunting them down in the name of the Emperor.

The Astartes will probably be the most solid base to follow the Emperor.

The High Lords of Terra may try to resist or work around the Emperor, but that won't last long since they are on Terra with the Emperor.

Individual governors would declare for or against the Emperor depending on their circumstance, whether they are well armed/trooped/far away/corruption.

But once the Emperor asserts his authority (at least of the Sol system), he will most likely find/heal/rescue whatever of his Primarchs are left, gather his legions of Astartes, and go on a crusade to properly reunite his Imperium.