Administrator's Name and Password

If you can’t do it through the gui (Graphical user interface). Try doing through the Terminal.

If you can't log in with your new password after restarting your Mac, take these additional steps:

1. Restart again, then immediately hold down Command-R or one of the other macOS Recovery key combinations until you see the Apple logo or a spinning globe. 
2. When you see the macOS Utilities window, choose Utilities > Terminal from the menu bar.
3. In the Terminal window, type resetpassword, then press Return to open the Reset Password assistant pictured above.
4. Select ”My password doesn't work when logging in,” then click Next and follow the onscreen instructions for your user account.

Restart computer in Safe Mode and see of the Admin Account appears ??

Is this computer Owned by a Company, University, School or Organization ??

There have been some reports of this situation where inexplicably the user account loses its admin privileges. This is just one of the reasons why people should always have a second admin account on their macs.

There is one fix that involves deleting a hidden .AppleSetupDone file. I am not at my mac to check the details but will report a little later.

Sherwin, I don't think this is the problem. The OP's account does not seem to have lost the password, the problem is that it appears as a Standard user, and not Admin.

Resetting the password does not hurt, but it is unlikely to fix the account status.

I looked up the process I had in mind, but unfortunately it does not seem to work in Big Sur anymore. I tried.

The idea is to delete the file that tells your mac that the initial setup was done; in the absence of the file, your mac will start as if it were for the first time, and ask you to create an account; that account is always an admin.

One is supposed to start in Single User Mode, and then mount the system drive and delete the /var/db/.applesetupdone

The difficulty now is that, at least in my mac, it does not let me mount the system volume, only the data volume, so the old way does not work.

You mean that there is no admin user at all is System Preferences->Users & Groups?