why doesn't administrator password work anymore

New problem - what exactly changed ?

Old problem - what has the user done to try and fix it ?

Reset Computer Password 

Intel Computer

1 - Restart computer and immediately hold Command + r keys

2 - Top Tool Bar >> Utilities >> Terminal

3 - Type resetpassword as one word and no spaces

4 - Press Return key

5 - Follow the prompts and remember the New Password

Apple Silicon M1 Computer 

1 - Choose Apple menu > > Shut Down.

2 - Press and hold the power button on your Mac until you see “Loading startup options.”

3 - Click Options, then click Continue. 

4 - Top Tool Bar >> Utilities >> Terminal

5 - Type resetpassword as one word and no spaces

6 - Press Return key

7 - Follow the prompts and remember the New Password