Is this a BIOS password you are having trouble with, or the Windows password?
If it's a BIOS password, you will need to call Dell support and verify with them that you are the registered owner of the system. Then they will help you unlock it.
If it's the Windows password, log in as the administrator (if you know the password) and reset the password for the account that is not working. Or if you are running Windows 8 or Windows 10 and using an online account, go to
www.live.com and click the "can't access your account?" link and follow the instructions to reset the password.