Damaged Macbook screen + External Monitor = Blank (literally)

I can't get an external monitor to work with my Macbook Pro with a broken screen. I made an appointment at a nearby Apple store to see what my options were nd luckily it's under a protection plan. The guys there hooked it up to some old monitor with an HDMI cable, but now I can't get it to recognize my monitor at home again (it worked briefly).. I realize replacing it would be the best route, but it isn't possible at this time. I've attempted to reset the SMRC myself, and they also did this at the apple store and it worked, but I'm assuming this was very quickly reversed somehow?

Is there any troubleshooting you folks would suggest to getting my Macbook Pro and samsung monitor to work together?