I need a password to unlock the disc macintosh HD

Here are the instructions if you forgot a password:

If you forgot your Apple ID password - Apple Support

If you forgot your firmware password:

Set a firmware password on your Mac - Apple Support

Welcome to the Apple Support Communities.

Please go to Disk Utility, and select "View" > "Show All Devices". Select each device (or entry shown), and take a screenshot of it and upload them all here. 

Jack

If you have changed the encryption password on your disk, then forgotten the password, your data have been successfully protected from intruders (and from you as well).

The default encryption password for your drive is set to your exact login password you provided when you first set up your Mac.