Watchdog to stop power supply
You can purchase supervisory chips that will provide a reset pulse if the WDT input has not been toggled in a certain length of time. For example, the Analog Devices ADM83xx series, but there are many others. The WDT will provide the pulse if the input gets stuck high or low, or simply fails to respond in time. 

You could also design a circuit that has that function, but the integrated chips are very small and not unreasonably expensive. 

Use a parametric search and look at the datasheets to find a suitable product if you want a fully integrated version. The general category is supervisory chips, but not all incorporate a watchdog. You can probably find windowed watchdogs too which will reset the system if the pulses come too frequently as well as too infrequently. 



If the reset pulse that the device provides (the above type has a number of options for pulse width) is not sufficient, you could stretch it with a monostable multivibrator chip. 

Interrupting the power to the subsystem is then merely a matter of connecting a suitable high-side power switch to the /RESET output. You may need to control the ramp-up current to keep from causing a glitch on the supply due to decoupling capacitance on the switched side, and/or you may need to actively discharge said decoupling capacitance to positively reset the circuit within the available time. 

Needless to say, this is a nasty bandaid fix and it would be better to solve the root cause first.