MacOS user password no longer working
Resetting User Account Password - This will work so long as you do not have a firmware/'EFI' password set on the machine. 

1) Boot into your Recovery Partition holding down command + R at the startup chime

2) Select Terminal from your Utilities Menu Bar

3) Type the following bolded command in: resetpassword

4) From the new window that appears, select your user account and proceed.

5) Type in the new password and confirm the new password

6) Click Save, close this pane and quit Utilities

7) Select your boot volume when presented with "choose startup disk" and reboot your machine