Almost no breast milk, help?

I have twins in the NICU that are 2 1/2 weeks old right now, so I have been pumping every few hours and taking milk in to them every day, in the past two days my milk supply has gone from just enough to almost nothing(and I mean nothing) I need help, manual expression isn't even helping! I really don't want to use formula but I don;t know what to do, I am a first time mom, please help.