Cannot display this video mode

In the past two days, my mac mini shows this whenever I turn it on: Cannot display this video mode. Optimum resolution 1440x900 60 Hz. I have to turn off the computer, press shift when I hear the chime, then release shift when the apple appears. After a few minutes, my computer starts. The next time I turn it on, I have to go through the entire process again. Help me, please! By the way, I'm very ignorant at most computer terms, so be kind. I often hear this from my children who are very good at computers, "Oh, Mom......"