Well, not a good day! Hopefully it was off, not being used, just got knocked off the table. HDD's are made to stand some physical abuse, and when off the heads are parked off the disks in a safer location. When on and spinning the heads are floating just above the disks themselves on air, much higher chance of physical damage to the platters. Happens all the time from external backups to college laptops.
Please tell us what kind of ext drive you have, and the situation, and height when gravity prevailed.
Afterward, have you tried to turn it on, to see if Windows still sees it as a bonefide working unit? And can you see any files on it? Also do you hear any clicking or noise coming from the drive? Might also change the USB cable, in case it was damaged at the same time.
Lastly, do you have a backup of the files, as on your computer or another HDD so they can be restored?