After discussion in the comments, it seems this is less of a case of you not being able to see your username and that the issue is that your username has changed. To change this back to bonaca, go to your profile page, click edit and change your username there. By the looks of things you problem may be caused by the information shown in the other answer to this question.
Additionally, the top bar was replaced a few months ago and your logged in username is no longer shown in the top bar. Instead, only your avatar, reputation, and current badge count are showed where your username used to be, and clicking on this information will take you to your profile, which will show your username.
For more information, check out the information here -> New top bar quick reference guide / Where has the link to <X> gone?