Call 888 407 4747 and tell them that you want to register. If you previously registered online then the Department of State has your information. Priority is being given to cases such as minor American traveling without their parents in Lebanon and medical cases. Once you are registered you will be contacted by when you are to go to an assembly point.
Try calling that number and see how long it takes for them to answer. I sure hope they have scaled up their phone bank since yesterday.
My retired ambassador friend got me registered directly with the special task force.