MacBook MiniDisplay to HDMI does not work
Most likely the adapter.  I have had cheap ($10) ones go out.

One solution is to buy the Apple brand ($30 ?).  I chose instead to buy two $5 replacements.  So far no problem, and the benefit of having extra ones is I can keep one at work, keep one at home without having to (remember to) unplug it, and also have one I carry with me incase I need to project somewhere.