Black Screen but external monitor OK

MacBook Pro 13" running El Capitan.

I often use my MacBook Pro in the office with an external 22" Apple Display, connected via Thunderbolt. I turn on the laptop, login, then once display is on both screens, close the laptop to use only the external display, using an external keyboard and mouse. This works very well. But yesterday I slept the laptop, disconnected the external display and power and took the laptop away. Now when I wake (or re-boot) the laptop it will only boot to a back screen (with normal chime)...but if I re-connect an external display - either via Thunderbolt or HDMI these work perfectly.

I have tried: a) Booting to safe mode b) resetting SMC c) resetting PRAM and none of these solutions work. When re-setting PRAM and holding down Option, Command, P,R, I get multiple chimes but still nothing other than black screen. Other than the screen, the laptop appears to be working perfectly - (caps lock light works) and external display is fine as stated above. Does anyone have any ideas?