Wd 2 terabyte external hard drive cant write to it

Does it appear in Windows with an icon and drive-letter?

If NO to above, there is a good possibility that the Boot sector has been corrupted, in which case you can verify by going into DiskManagement (Rt cliick on my computer, select manage, the select disk management (rigth pane). Look to see if drive shows up. If it shows up, then check that it shows that it is initialized and healthy. If it shows unallocted, you would have to reformat - BUT this would wipeout any data that is on the drive.

Another option is to download WD's utility disk and verify that the drive is still good.

Is it possible that the drive got bumped while it was writing, as this can cause corruption.

WD 2TB drives do NOT have the best track record for reliability.
You did not specify which model.
2 TB blacks 1/2 egg (indicate problems) ratings is 23 %, and
for their 2 TB Green Models (which I would never buy), 1/2 egg ratings are a dismal 33%.
Ref: http://www.newegg.com/Product/ProductList.aspx?Submit=E...