Mac Pro won’t accept password even when reset

See if any of the following articles can help:

• Can't Log In to Your Mac? Create a New Admin Account - Lifewire
• Change the Admin Password with Mac OS X Single User Mode - OSXDaily

You can also reset the Administrator password by using Recovery Mode, as follows:

• Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
• When the menu bar appears select Terminal from the Utilities menu.
• Enter resetpassword at the prompt and press RETURN. Follow instructions in the dialog window that will appear.

Once you reset the Administrator password, restart your notebook, and then, log in as the Administrator. From there you can reset any of your user account passwords.

You could try adding a new admin account and see if you can log into that.

User Account – Add New Admin In Single User Mode