The network accounts are related to open directory, but to show the "normal" MacOS login page, you don't actually need to disable it. Go to System Preferences -> Users & Groups. You will see this image (except your Username will be highlighted):
Click on Login Options. If the padlock bottom left is closed, almost everything will be greyed out (like it is now). If the lock is closed and you do not have administrative privileges on your computer, you will want to talk to your IT department. If the lock is closed but you do have administrative privileges, click on the lock to unlock the settings by typing in your administrative username and password.
Once the selections are no longer greyed out, change the "Display login window as:" selection from "Name and password" to "List of users".
EDIT based on subsequent information:
Let's assume you can't get to System Preferences because you can't get into your computer. We then follow these steps.
Do you have a local user account or a Network account? If you have a local account, you should be able to type in your username and password to get past the prompt. If you have a network account, try connecting to wifi (I think there is a wifi icon in the top right corner you can use without logging in, or plug in an ethernet cable. Then log in with your network account. If you have NO account information (you were never assigned/told/asked to create a log-in or password) we can't help you - you'll have to wait for the IT guy to get back.