The external hard drive's read/write heads may have "crashed" into the data platters when it fell down - this results in drive failure and the drive can no longer be used. That's why it cannot be formatted.
If you want to test the drive to make sure it has actually failed, test it with SeaTools for Windows
which can test any brand of hard drive. Click Downloads
on this page: http://www.seagate.com/support/downloads/seatools/