It was during my first job as a computer technician in high school that I developed the maxim: first check power, then check clocks.
When you approach a repair job, first check the power supply. Not even a machine in perfect working order can run without energy. Restoring power is your highest value repair because a power failure is so common, so complete, yet so easily remedied. A live power receptacle, a new electronic power supply, fresh batteries or a tank of fuel are all within easy reach.
Sophisticated machines cannot run without a clock to synchronize their operation. Only the power supply will work without the clock. Digital electronic circuits march to the beat of a crystal. Steam engines pulse to the rhythm of their valve gear. Check that all subsystems of the machine are receiving their timing signals.
It’s embarrassing how often I forget this rule myself. I’ve wasted hours troubleshooting a machine whose power switch was simply turned off. Did I mention that you should first check power, then check clocks?