Well, pH probes aren't rocket science, and their design is pretty simple. I've abused pH probes worse than anyone on here ever will be able to, and they (mostly) survived. About the worst thing you can do to them is let them dry out all the way. I doubt there's anything too wrong with the probe itself. A quick calibration and it will be semi-accurate again.
Regarding the solenoid, if you unplug it, does it click and shut off your CO2 flow? If so, it's not the solenoid.
That being said, if the meter was reading 5.5, accurate or not, and was set to go off at 6.5, then you know that the problem was 'downstream' of the probe itself, either with the solenoid or the monitor software/hardware sans probe. Check to make sure everything's on/off. If that fails, maybe try turning it on/off or resetting it (depending on what your model can do).