I've been using my RaspberryPi headless, connected to a USB hard disk and the network from the cupboard where my router lives. It was running for 21 days like that, no problems, then I re-started it. Now, bizarrely, it will only boot when I take it into the living room and connect it up to the TV with HDMI. I have tried all the available power supplies, so I don't think it's that, and simply un-plugging the HDMI cable is enough to stop the RPi from booting.
When I plug in power, the red light comes on, then the green OK light flickers for a couple of seconds. If the HDMI is not connected, that's all it will do. Network lights don't come on, I can't get anything to tell me what's happened. With HDMI, the network lights come on after a tiny pause and the green OK light keeps doing its thing.
I have tried different SD images, no good. I have tried booting with nothing connected, no good.
What can I do? It's not practical to keep the RPi where it is currently working - any suggestions? :