I saw mention of this here on MacRumors several years back, but no suggestions for a fix.
Summary: if my external display (30" Apple Cinema) is attached to my MBP (by both Thunderbolt and USB connections), the MBP won't boot (or restart); it makes the startup sound, but nothing appears on either the built-in or the external display (both remain black). If I disconnect the USB cable (makes no difference whether I disconnect the Thunderbolt cable) and power cycle it, it does boot, and when I reconnect the USB cable, the external display comes on and everything is okay.
This is a new problem, but I'm not sure how new. It might have started with El Capitan (which I'm now running), but it might have started before that.
Summary: if my external display (30" Apple Cinema) is attached to my MBP (by both Thunderbolt and USB connections), the MBP won't boot (or restart); it makes the startup sound, but nothing appears on either the built-in or the external display (both remain black). If I disconnect the USB cable (makes no difference whether I disconnect the Thunderbolt cable) and power cycle it, it does boot, and when I reconnect the USB cable, the external display comes on and everything is okay.
This is a new problem, but I'm not sure how new. It might have started with El Capitan (which I'm now running), but it might have started before that.