After the Mac boots up,
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.