The phrase 'run out of steam' means that one has lost the energy or enthusiasm to keep doing what they were doing.
Example in use: "The student campaign ran out of steam when it became clear there was no way they would win a seat in the council."
Interesting fact about Run Out Of Steam
This phrase originated from the idea of an old-fashioned steam engine that steadily slows and then halts when the fire that controls the boiler is too low to produce steam.
