You can try the following. Download the tool USBDeview (Freeware):
http://www.nirsoft.net/utils/usb_devices_view.html (scroll down for the download links)
When you run the program it will show all USB devices that are currently attached and have been attached in the past. There is a column called VendorID. Click on its name to sort the list and look for devices with the ID "0c70". This is the ID for our devices. Delete all entries with this ID by selecting them and clicking on the trash can icon in the upper left corner. Just confirm the request to delete the entry.
When you now unplug the USB connection of the pump and plug it in again a new entry for this device should appear (ID 0c70 again). Currently connected devices are also highlighted with a green background and have a green dot in front of their name.
If the device entry is completely missing I can only assume there is a problem with the pump, the USB cable or your system. If you own a multimeter you can do a continuity check of the cable. If you have a second PC available you can also test the pump there. Running it try for a few seconds is no problem.