If the drive is locked, it can be locked in two ways.
1, Dell has locked it in the BIOS with an admin password. If this is the case then there maybe a section in the user manual to cover it. If not, then you will need to contact Dell support for help.
2, The drive itself is locked with a password. If you are sure you didn't set a password, then this has either been set by the vendor you bought from, or the drive is not new and the pervious owner has set the password. In either case, if you can't find the password then you will need to have the drive replaced.
The most likely of the two I listed is, Dell set an admin password in the BIOS.