I had just refilled my spa (Sundance Optima 1995 with Sentry 800 control) and turned it on to start heating it. It had been running for about 24 hours and I came out and it was silent. The display just has four solid dashes in it ----. None of the buttons work and nothing seems to function. I turned off the power to it and back on and it remains the same. I tried unplugging various things from the computer and see what might change things. When I unplug the sensors and turn the power on then the display looks more normal but flashes various sensor warnings, and none of the buttons work (expected). When I plug the sensors back in then I just get the four solid dashes. So I would expect the problem is related to my sensors, but I don't know what this means or how I troubleshoot what the exact problem is.

Your circ pump may just be airlocked.