step 1:after switching ON the computer enter cmos set up and check the status of USB. If found disable then enable it and press F10 key from your key board. When prompt to save then press y for yes and click ok option.
step 2: right click on my computer and go to manage from context menu. Now go to device manager and check the status of USB driver. If you see some no of ports then there is a problem in your USB driver.
step 3: right click on your USB drive and select property. Now click roll back the driver.
step 4: scan your pen drive it may be virus effected or try to open it from windows explorer.
step 5: If not solved then install USB driver form the CD of mother board provider.