Computers don't decide the password is wrong all of a sudden. The only time I have ever seen a password trully not work was in a BlackBerry phone where the encryption would accept the password, but not unlock the device. 99.99% of the time, the password is actually being entered wrong, or the right password but for the wrong thing. Maybe you are using your bank password or something. Can check with WD if there is a bug in whatever you used to set the password (assuming it's not just a Windows user password, that is trivial to get around).
Check caps lock, make sure all the keys are working OK. If the drive was encrypted there is probably no way to get data back unless you made a key recovery disk.