USB flash drives, camera's, mice, printers and any other USB equipment can draw power from your USB port. If the demand for power overwhelms your USB port, it can shut down your USB port or ports completely. Most PCs have multiple USB root hubs, in which case you can try moving one or more devices from one root hub to another (From the back of the PC to the front is generally worth trying). The other solution is to purchase a powered USB hub!