Power led won't turn on

Hi, This is a non-urgent question. It's just a power led not lighting up. It's a friend's PC. He told me it used to power up when the PC was new, but after he took it to a service tech it didn't turn on when it came back, neither did the HD led. Well the HD led did turn on, it was just that it was not placed where it should be (it was inside the case). So, getting to the point, I...