Not a 718, but most digital electronics run on the same principle, so this is what happened to me recently.
When modifying my cheap signal generator for an external 50MHz reference, I noticed that the front panel would go unresponsive when I would interrupt the microcontroller clock signal. Even shifting the frequency would cause the micro to lock up. Power cycling was the only thing that returns it to normal, and the power button only worked when the clock signal was restored. When I want to use my counter, I need to power the GPSDO first, because if I power the sig gen first, as the GPSDO comes on, the sig gen goes unresponsive and must be power cycled once the GPSDO is locked. Changing reference frequencies is enough interruption to cause the issue.
If the power button doesn't come back, it might be that the clock signal is dropping out completely after a certain amount of time.