I forgot imac lockscreen password

I have forgotten my lockscreen password & I have tried to reset it by going into cmd + R setting & using the terminal/cmd box to reset it. However when it pulls up the boxes to select which user to reset password for I cannot make any changes to anything. This used to work but now does not. I wonder if I put some master password into the system or encrypted the hard drive in some way but I don't remember doing this.