There could be a number of things going on here.
Sounds like one of your automated data management programs marked all these as marketing suspended, so you will have to locate and list out all the triggers that would activate this to discover the cause.
In terms of the changing them all back to marketing suspended false, I am wondering if you have a safe guard against running 2 million people through a smart campaign at once, your system could be aborting the campaign. You can check that in the admin > smart campaign restrictions.
Take a look at the activity logs to see what happened and why they got suspended. Is there a Marketing Suspended Cause? Did the emails all bounce? What type of activity preceded the suspension?