Hello,
Do you mean that the CPanel menu item does not show up in your CPanel? OR is it just your particular account whose name has been changed?
If the latter, you may want to check the database name / user. CPanel, by default, prepends each username + underscore before each db/username. (e.g. If my username is "blaw" and my db name is "mydb", then the real db name is "blaw_mydb", so if you change the username to - say "shoide", then the database gets renamed as "shoide_mydb"). HTH.