How do computers output a signal without an input?
Software and circuitry is just a bunch of switches (transistors) that trigger each other in a big complex loop so they continue to trigger each other. 

You manually crank an engine to turn it over and start it, but what makes the engine continue to spin after that when you stop cranking? The engine does, of course. It keeps itself spinning by hitting all the right valves and switches while spinning to keep spinning. A processor is the same.

Also see:
What happens at hardware level when we feed a code?