When the device is connected to the computer, in the taskbar click in the icon which shows that a device is connected (the one where you can "Safely remove" the device) and right click on it and select the show devices and printers option. When it opens, you will see all the windows phone devices you have connected to the computer. Disconnect your device and uninstall it and its drivers by selecting the appropriate option after right clicking on it.
Connect it again, it should now install its drivers again.
And just in case, if you have the correct date/time on the device, just confirm they also match the region/location set in the device. for eg- suppose you have set the time to indian standard time (IST) but the region of the device is set to that of USA or some other location.