Firstly, I think you should check your device manager and see if the drive is found in there. If the drive has a red cross through it, try to uninstall the device and reboot your system. If the drive is not found when you reboot it looks like you've had a flash failure and so you will need to recover it using the tools provided by BTC.
and download the flash recovery tools. Also read throug the user guide to help you get the drive back. It could also be an idea for you to download the latest version of MTKFLASH 1.80.1 from the web to help you flash your drive in dos rather than in windows.
What you'll have to do first of all is to make a bootable floppy, and then using the guide and the tools, flash your the firmware of the drive through DOS. This should (upon restarting) automatically reinstall your device and sort you out.
From my own experience with problems with the windows-based flash, I would personally look for the .bin firmware upgrades for the device - such as those made by the dangerous bros. By using the same procedure you can update the firmware of your drive regularly- hopefully with less problems by using DOS. For myself (who owns the same drive) I intend to merely use the liveupdate as a source of new firmware available, and find the .bin file to flash it in dos.
Hope this helps.