First, I would find out where all of that fluid went, otherwise it will just happen again. Check for any wetness leaking out of the brakes or master cylinder, or see if you blew a brake line somewhere. If you don't find anything suspicious try putting some fluid in. Next time have someone watching underneath for any brake fluid squirting out anywhere when you push the brake pedal.
If it went completely dry you will have to rebleed them.
Aaron
