If you think the problem coincided with plugging in the USB device, I would think the power supply could be marginal. It's also possible that you hit the heat threshold and just need to do some dust removal from CPU and other fans. What's the configuration of the PC and size of the PS?
Yesterday was simply plugging in a usb cable to download some pictures and the power suddenly shut off. Would not turn back on no matter how many times I tried. After about 10 minutes, it turned back on, and then 10 minutes later cut off, and will not come back on. Haven't had a chance to call Cyberpower's tech support. Any quick ideas?