Steve,
I had exactly the same problem back in 2014 with my 798c after upgrading to version 6.3. I stopped seeing fish. At first I thought it was me, or the settings, or that my lake just sucked. Eventually I contacted Humminbird customer service via email at the time and they were able to tell me the version of software (and provide it to me) that my unit came with by looking up the serial number. Their first answer was that they don't recommend downgrading due to the risk of "Bricking" the unit, but my response was that I understood the risk and was willing to take it on (as the unit was useless to me since it wasn't marking any fish).
Not that this helps you, but the version I downgraded to was 5.84, and I have stayed with it ever since.
Good Luck!
Kevin