Unable to open the lock for making changes

Your account was changed to Standard, you need to make it an admin account, which requires another admin account.

See if you can create a new administrator account by restarting the Setup Assistant:

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Be very careful to notice the spaces in those Terminal Commands.

Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will stile there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

Has it worked for you yet?

On that last command, HarishPrabhu_02, you appear to have forgotten the space between the command name and the option ‘-uw’. (You also forgot the leading slash, ‘/‘, in the path to the command.)

👍

Or use the Harish P username & password in that dialog box.

Great news, take care my friend. :)

Login to the new Harish P user & from there grant the Admin admin rights... change it to Admin account.