Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
'Black screen' cases are almost never recoverable, and you have already tried the right things.
Tell me your zip code, and I will list for you the nearest Garmin facility to send it to for repairs.
(How old is the Garmin 192c in question? And BTW if it is out of warranty, I can quote you the flat rate Garmin would charge for repairing it.)