The monitor was working one day and the next day, it would not come on, screen black. My grandson has had the monitor for almost three years.

Any power light ??
Anything you could add to help diagnose the problems ??
What kind of monitor, model number, size.

Hi,
I'll hope for the best and assume the simple fix first. Check the plug at the back of the monitor to insure it wasn't pulled out slightly. Sometimes, moving something around can cuse the plug to lose proper connection.
Also, check the power source to insure that you have power to the unit. We had a lightening storm recently and the power strip surge protector did it's job. I turned it back on and everything powered up again.
I hope this helps... Good luck!