Why does my MB display only work when external display is connected?

I've had this issue for quite some time now, and it's been happening at irregular intervals. Sometimes when I used to switch my laptop one, after I see the login screen the display goes black completely. However when I start the laptop connected to external monitor the laptop and the display works perfectly fine. Now this happened again today and I read that it might be a software issue or that the user files are corrupted somehow so I decided to format and install a fresh clean install of mac osx. I ran the format using the laptop's display, without using my external as everything was working fine. Now after the install is complete the same issue is happening again, I just get a black screen after I see the Loading bar.

Any ideas what might be causing this?

Thanks!