Message on the screen requires to turn off computer, freezing it
My computer (18 months old) seems to be working normally but after several minutes a sign in the middle of the screen appears directing me to turn it off or to re-start it. This messages freezes all activity in the computer.
The message appears in English, French, German and Japanese.
I don't have insurance and the Apple service center wants $450.00 to fix it. Can anyone help?