The screen on my dell laptop sometimes goes black if I leave it idle when I move the mousae the screen does not come back on. I have to turn the power off and then back on losing what I was working on. How can I stop this?

Sounds like you need to change the power settings so the screen doesn't turn off.
I've had a few laptops do this, they simple refused to wake the screen after it was turned off like this, but disabling that option worked.
The power options are generally located on the control panel.