I lost all my data when I changed username

You must have skipped a step or performed a step incorrectly. Here is an Apple article with instructions for changing the user name of a macOS user account:

Change the name of your macOS user account and home folder - Apple Support

You must perform each step exactly as Apple has described. Any deviation except for the actual name used will result in failure. Also, make sure the new user name does not contain any spaces.

Use the Go menu -> Computer

Double click users. The other username you were logged on hopefully is still there.

As long as you remember the password for it, just select the Apple logo menu to log out, and then log in the other username.

If you don't remember the password, see these directions:

https://support.apple.com/guide/mac-help/change-the-login-password-on-mac-mchlp1550/mac

If the other user isn't there, you might find some files by keyword using Devon Technologies EasyFind:

https://www.devontechnologies.com/apps/freeware

If Easy Find does not find them, locate a data recovery expert who works on Macs. Many are on http://www.apple.com/buy that do Mac support also know where there is good data recovery.

When you say you can't access them, are they in the folder of the currently logged in user with the house icon, or a different folder?