I recently had a user that when he opened up Quickbooks, he could only log in as admin. The username box to enter another user account was not shown.
The issue with this is that the qbw32.exe service gets hung and even though the program is closed, the service is still running.
To resolve this problem, close Quickbooks and then go into the Task Manager>Processes tab, right click on the qbw32.exe service and then click End Process.
After doing this, open up Quickbooks and the username field should be visible again so you can log in as another user.