I made a rib roast last night and it turned out to be way underdone in the middle. I refrigerated it. Can I reroast it?

I would also recommend a meat thermometer.

Yes. Just bring it to room temp before going back in the oven. About 30 minutes per pound. So, go from how much you already cooked it & add time as needed. (It will continue to rise in heat about 10-15° after its removed from the oven.)