Those species are usually hardy to about -5F. You may have lost some culms, but if you're seeing life below the mulch you're probably fine; I would guess that you have a good chance of their recovery. Some fertilizer now or in April shouldn't hurt. I tend to use controlled release fertilizer, like Osmocote. Good luck.

I wouldn't worry too much. I've experienced the same culm die-back with new plantings of Phyllostachys species. My P. rubromarginata and P. nuda died to the ground their first winter. They had the same green bases to the culms below the mulch like you described on your Fargesias. Mine sent up a bunch of new canes and were as healthy as can be that summer. Once you're certain the old canes are dead (i.e. they reamin brown and don't re-leaf in early summer when new shoots emerge) cut them off above the green portion at their base.