Macbook Pro external monitors in clamshell mode

I have a current-gen 15" rMBP with the Nvidia graphics card. I have 2 external monitors connected via HDMI with an adaptor into the Thunderbolt ports.

Up until yesterday morning I would sleep the Macbook, wake it up with a mouse click, and the monitors would work; now, however, they only receive a signal if the Macbook lid is open. As soon as I close it, the monitors just say no signal found, no matter if I click the mouse/keyboard in case the Macbook has gone to sleep.

Any ideas?