Undermount Bath Sink Dropped
It was glued to the underside of the counter with no hardware to retain it? Did you have it installed by a "professional" countertop installer or a plumber? If so, you need to give them a call.
Normally with stone/granite/marble tops, undermount sinks are retained by a few clips that are inserted into milled pockets in the underside of the top, as you mentioned. They're siliconed as a good measure, but probably more for the purpose of sealing leaks.
I doubt that silicone adhesive would sufficiently adhere the sink to the underside of the counter. I can't think of anything else that might be appropriate for this application. Perhaps a countertop company in your area could recommend a better product.