Cant extend display; one screen is always dark/off

[kaimi]

I have a Lenovo E545 laptop
Processor: AMD A6-5350M APU with Radeon(tm) HD Graphics
Grachics card: AMD Richland Radeon HD 8450G
I'm running linuxmint 17.3 64 bit

I'm trying to set up an extended display across my laptop screen and monitor. When I use the display application to set it up, only the monitor or my laptop screen will display depending on which one I set as the primary. What is odd is that the desktop IS EXTENDED. I can move my mouse across the screens or move windows from screen to screen; it's just that the other screen is completely off and I can't see where my mouse or windows are on it.
I'm not saying that the screen is dim; it is completely off or isn't displaying anything.
If I press my windows+P key the display may change to:
1. both screens not displaying anything
2. only one screen (not primary) on display, may or may not be the correct resolution
3. only primary screen is on display, may or may not be in the correct resolution
and I can cycle between combinations of these every time I windows+P
Hope someone knows how to fix this; it's the only obstacle in the way of my liberation from windows 10. Otherwise, linuxmint is pretty great

[vimes666]

Try using another monitor cable.

Did you do the setup through <control center><monitors> or with anothe application?

If you used <control center><monitors>, did you click 'detect monitors'? You can also set your resolution and refresh rate here per monitor, and you can drag them about. There may be a setting that your external monitor doesn't like. Do you have the same image in both screens when you tack 'same image in all monitors'?