Have you tried this, open Device Manager on your computer to reinstall the driver for your device while your device is connected with your PC via a USB cable.
Microsoft Windows Vista and 7 users
To open Device manager on your computer
Click on the Start button.
In the Search box, type device manager, and then press enter.
Microsoft Windows 8 users
To open Device manager on your computer
Click the Desktop Tile to open the Desktop.
Click >> at the bottom right of the taskbar.
Select Control Panel from the Menu.
Select Device Manager from the list of control panel items.
Note: If you do not see the screen with the Device Manager option, click the View by drop-down menu and select Small icons.
Microsoft Windows XP and 2000 users
To open Device manager on your computer
On the desktop, right-click on My Computer and click Properties.
In the System Properties window, click the Hardware tab.
In the Hardware, tab click the Device Manager button.
To reinstall the drivers for your device
Scroll down to the Universal Serial Bus controllers (USB section) of the Device Manager window.
Look for an MTP Device: Media Device, Unknown Device or any other reference to your phone. Click on this entry and choose "Uninstall" whilst leaving the device connected.
Wait a few minutes, and the computer will reinstall the drivers for the device.
Now, the phone is accessible via Computer/My Computer on your computer.
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