why can't I get my macbook air to open up?

What do you mean by open up? Lift up the screen part of it or actually getting it to turn on?

If it is that you can’t lift up the screen, then check for dirt and other things stuck in the hinge.

If it is the more likely option, you can’t get it to turn on, then connect it to its charger and wait an hour or so, then try pressing and holding the power on button, and see whether it turns on.

:)

Keep in mind that the password used to access your Apple ID may not be the same as the Administrator password you created for your Mac.

Most upper right button