Hi there. I am sorry to hear about your issue. It seems like you haven't disconnected the drive properly which has caused this issue. What model is your HDD? You can run chkdsk by following these steps: Press WindowsKey+R, write cmd, then write chkdsk /f h: (replace the 'H' with the drive letter of your external drive).
You can also try assigning a new drive letter. You can do this by right clicking on the external hard drive and then clicking on Change Drive Letter and Paths. Then you can click Change to alter the current drive letter and then click OK.
If this does not help you could connect the drive to the computer, copy the data and then format the drive and re-copy the files.
Hope this helps.
Cheers, Dorian_WD