Are you really in VA and if so why are messing with the plumbing in the early AM?
If it were me, I would ask myself, does it drain at all?
If so, I would pour, as already mentioned, some lime scale remover or similar product into the drain and let it sit for awhile and see if that frees it up.
If that doesn't work, I would take a pair of channelock pliers and see if I could turn the stopper and release it.
Failing that, I would just remove the whole assembly and deal with it at my leisure.
I looked at the installation PDF and it is a standard procedure for any sink drain.
The removable pop-up cartridge is probably just a metal spring assembly which I would expect to fail after a time from corrosion or trapped whatever.