My macbook does not let me in to system preference changes, after I tried to change user name once...

Take it to an apple store. These things are difficult to diagnose without seeing the screen.

Did you set a firmware password? I never have, but it asks you for a password every time the machine boots.

You need to get your camera out and post pictures of what you see and do when you boot up in single user mode. Once you have transfers the pictures to your computer.

click on camera icon.

another window will appear.

click on Browse button

An open file panel appears. Select the file.

After selection a file, I have this screen showing.

You will have to click on Insert Image. You may have to scroll down to see Insert Image button.

On Windows it's more complicated. Here is an outline of one way. There are others.

is always empty (see picture)

I don't see the picture. Place in google docs or dropbox & provide a link or post here. Click on the picture icon above.

You can use this procedure to copy your document to a flash drive.

Do you have backup? if no, and you need to recover your data. See this link:

backing up from the command line via single user mode.

https://discussions.apple.com/thread/8101803?answerId=32357328022#32357328022

Get the Mac to set up an additional administrative account. You can then change the password on your old account. This will work all all releases of Mac OS X so far.

Start with your computer power off. Hold down command-s. Power on your computer.

Type in the following:

The first two commands will depend on your release of Mac OS X. Look at what is typed out in the console to determine the exact format.

# Type the follow two instructions to access the startup disk in read/write. Press return after each command.

# in case of partial success repeat this command until errors go away.

/sbin/fsck -fy

/sbin/mount -uw /

cd /var/db

pwd

#List all files. The l is a lower case L.

ls -a

#The move command acts as a rename command in this format.

mv -i .applesetupdone .applesetupdone.old

# reboot your mac

shutdown -r now

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

Limnos adds detailed explainations:

http://discussions.apple.com/message.jspa?messageID=8441597#8441597