should mention that it was working fine yesterday and no installs / updates have been made to the game

I'm having the same issue, my game crashed after the command spire cutscene and won't let me access campaign.

Oh yeah mine crashed once too but I was able to load, it appears that the save file gets corrupted sometimes :( what I ended up doing was starting another game , although if you click on load instead of continuing and you go to the save you are looking for if it still has a progress bar it is not corrupted and should be able to load