Hello and welcome.
Sorry about your issue. I don't know of any shortcut for the Return key when logging in. You can use the mouse/trackpad to click the circle with arrow in it on several versions of OS X, if it is there. It is usually to the right of the password dots in the box, just to the right of the password box, or at the right of four buttons below the user/password boxes and labeled Login. Or, you could attach an inexpensive USB keyboard and use its Return key.
-Which version of Mac OS X?
-Which model MacBook?
Unfortunately, it will require repair to restore it to full functionality.
C