This issue can be caused by several things. If you are running Windows XP, then in order to fix this, you should check the following official website by Nokia, where a solution to this has been provided. You just need to follow the steps provided. If you are using Windows 7 or Windows 8, and your device isn't recognized, you could try to right click on My Computer > Properties > Device Manager > right click on the Nokia device with a yellow exclamation mark on it and select Uninstall. After you did this, reboot your PC and try to connect your device to it again. You can find more information about this on the following forum page.