Without knowing which operating system you are using with your "new" Mac screen and assuming you do have an operating system? (An iMac or a Mac mini?) Go into System Preferences, found in the tool tray as a cogwheel in a box and in there you will see a keyboard and also a mouse icon click on each one in turn and follow instructions after checking batteries in keyboard and mouse. Without knowing more about your setup that is the best I can do. Just realised that I could be on the wrong track altogether - if so SORRY
Very best regards from Keith in Derby England.
So you failed to prepare -- now be prepared to fail.