I took my PC desktop in for upgrade over 3 months ...

I took my PC desktop in for upgrade over 3 months ago and they did a pretty good job. 2 days ago I took it in because of a power fluctuation kept causing my monitor to shut off, but my desktop was still running. Guys knew what they were doing, the tech told me it was a power cable ( that they installed with the upgrade) that was connected from the power converter to the rest of my computer. To my dismay they charged me for it. This should of been warranty work considering it was only 3 months old and they installed it that way.