Can the wrong power cord harm a desktop computer - my daughter used a TV power cord and the computer will not turn on. Thanks!
If you are only talking about the passive part of the cable, i.e. this one:
(or similar designs), then no, or at least vanishingly unlikely.
If you are talking about the transformer portion, i.e. this part:
then perhaps, but the connector should most likely not fit if there would be a problem (but that is not fail-safe). These external adapters are more or less exclusively used for laptops, but there are desktop computers that use them. Thought I'd mention it since we don't know exactly what kind of desktop computer (or TV, or cord) it concerns.
With "perhaps" I mean that if the voltage rating doesn't match what the laptop wants, then it will not work, but if it goes as far as damaging it or if it has safeguards protecting it against things like that is difficult to say without more information. The first thing that would stop functioning would probably be the adapter in itself.