USB Port not working sometimes.

A few days ago I reinstalled my windows 7 because my pendrive was not detecting (not detecting = no power in usb!!!). When I reinstalled the windows 7 it did not work. So I thought that the drive is kinda dead. So I got myself my friend's pendrive to check but his pendrive also didn't work. But sometimes in between it started working. And then again no power in my friends usb. So my friends pendrive works in between sometime. So what should I do??

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