Without the password you would have to reinstall the operating system.
What is the operating system ?
Do you mean that the operating system session is locked ?
If it is windows, it can be unlocked only with the logged user password, or an administrator (all unsaved work will be lost if you switch user to unlock it with an administrator account).
If it is linux, you will probably be able to unlock it by switching to any other user account (all unsaved work will be lost if you switch user to unlock it).
Yes please provide more details to get the assistance you need. If this is a BIOS password, what type of system is it? Laptop? Desktop? Thanks.