Why won't my monitor wake from sleep mode?

I was recently given a brand new Acer laptop and took it out of the box just a few days ago. Everything seemed to be working fine until I put it into sleep mode. When I tried to wake it up, the monitor stayed black. I know the computer itself is awake, though, because I am able to type in my password, hit enter, and hear the “log in” sound. But I can’t see anything! The only solution I’ve found is to hold the power button for ten seconds and then turn it on again. This can’t be good for the computer.
Does anyone know how to fix this?