that would be a blinded ovum, in wich the the gestational sac and placenta develops but the baby never actually developed.
i had a blinded ovum once wich showed an empty sac at 7 weeks, then again at 9 weeks and at 9 weeks 1/2, the sac actually kept growing and also the placenta and at 10 weeks i started misscaryng and had a D/C
By 11 weeks (depending on how far along you were when the scan was done) you would have seen the heartbeat and fetus, not just a sac. If you didn't, your doctor should have talked to you about what they wanted to do. You can request another scan just to make sure there is nothing there. If there was no heartbeat or growth, your body may or may not pass the embryo. If it does, you'll bleed (not sure for how long. All women are different) and pass everything. If you don't, you would need to get a D&C to get everything taken out to prevent infection. After that you should get a follow up to make sure they got it all.
if its an empty sac good chance it could be a miscarrage.. wait a week and ask for another u/s