After the Mac boots up,
go to the Control Panels under the Apple menu in the upper left of the desktop. Select the Extensions manager. When that window opens, check at the top of it and make sure the proper extensions set is selected. You might also try booting up while pressing the Command (the key with the Apple on it), the Option, P and R keys. You can press the power button and then quickly press these keys together. Keep them pressed until you have heard three startup chimes. This will clear and clean the startup preferences for the Mac. We call this zapping the PRAM. You might also rebuild the Desktop by starting while the Option key is pressed until you are asked if you want to rebuild the desktop.