LostinAlabama,
The SI units are great, please take the time to go thru your installation and what you are seeing. The folks here will help you get it installed.
Do a direct hook up to a charged battery and turn the unit on (red = positive side of battery), does the unit come on?
Something as simple as the power cord could be an issue if the unit does not come on. You can check the power being supplied with a cheap voltage meter. Also, if there is a fuse it could be blown or have a poor connection.
If you purchased a used broken unit. then you either need to go back to the seller or contact Humminbird Customer Service, they really do great work. To repair the 797C unit (if it is out of warranty) costs $188 and some change (it is still worth the repair cost).
I hope you get it fixed, you will enjoy it.
George