I have updated this post today.
I received my unit back from repair, no trouble found, with a new transducer, which I did not need and was expensive at a repair fee. I had tried 3 different transducers so I knew my no sonar was not that. I did some further testing.
Originally I thought the problem was related to have two software bin files on my SD card and after replying NO if I wanted to update back to earlier version, then my unit stops at snapshot screen after it appears to have finished loading. You have to exit this screen and then there are NO SONAR signals.
After getting some nasty posts about how stupid I was for having two bin files on one SD card and it was my fault, for something as benign as two files of the same type on one SD card. That could only be if the hardware/software unit (Humminbird) was "dumb" enough to try to use or confuse two files during loading.
The following was performed with two SD cards, 1 bin file each, to prove my point about two files could not cause this problem.
I backed up to the beginning. I restored defaults, reloaded ver 6.64 software, which is where I started.
I then put in another SD card with only software 6.74 on it and put it in the unit. This is what I originally did but did not install the software right away on the next fishing trip. So I had to reply NO to an update question about upgrading from v6.64 to v6.74. The unit does the same thing, it appears to load, ending up at snapshot screen. No sonar signals. Since I had not updated the software, but replied no, I started down a path on trying to figure out why my transducer was not working, I had not changed anything. Even after upgrading to v6.74, my original SD card did now have two software versions on it, which was not a problem but leads to the upgrade question during startup, wherein the problem really lies.
So, the final net of my problem, at least for my unit, is that if the unit finds a SD card with any software file that is not the current version in the unit (only needs 1), and you rely NO to upgrade to skip the file, the unit ends up with no sonar, transducer does not work. I can see if one puts one new software version on the SD card, making sure it is clean of other software files, immediately installs it, then you would never see this situation.
I do not know if this is unique to my unit or can be recreated on other units. My take is there is some type of potential software "bug" in processing the NO to an upgrade request.
The pics are of a sequence where the unit software and SD card software does not match. You reply NO to upgrade, end up at the the snapshot screen, you have to exit out of this and then the sonar signals are all blank. The last pic is after taking out the SD card, thus avoiding the upgrade question, and then you can see that the transducer (original one) gets signals.