|Most likely the drive has failed and will need to be replaced. Others may be able to help with recovery, if possible.|
You really need to have a backup of important files. Having only one copy of important data is asking for trouble. Hard drive fail, often without warning or apparent reason.
External drives, particularly in the larger sizes, seem to have reliability issues. But this isn't always the drives fault. The rugged exterior hides the fact that they are really quite delicate internally and they are often (quite unintentionally) mistreated.