Troubleshooting the USB ports and peripherals on a Thunderbolt display

I use two Thunderbolt displays with my mid-2012 retina MacBook Pro. I keep my iPhone 5 in a dock hooked up to the left Thunderbolt display via USB. On Friday, I noticed that when I went to commute home, my battery was very low. Odd! Today, I noticed that sitting my phone in its dock didn't make the charging lightning bolt show up next to the battery indicator.

A few minutes of testing proved that all three USB ports on the left Thunderbolt monitor are failing to charge my phone. Using the same cable in the other Thunderbolt monitor, or in the laptop itself, behaves exactly as expected.

How do I go about troubleshooting these USB ports?

UPDATE: OS X (10.9.2) also does not see the camera on the left-hand Thunderbolt display, and won't send sound out through it, either. It's not just the USB ports, clearly. How do I troubleshoot, uh, everything-but-the-screen?

UPDATE 2: From a suggestion in the comments, here are the System Report details on USB, Cameras, and Audio:

I never figured out what the problem was, but it went away when I reset the SMC.

I regularly experience the USB ports and FaceTime camera not working with my Apple Thunderbolt Display. Although I've reset my SMC, it doesn't always restore the USB functionality.

However, I've noticed that after the display has been running for a few hours the USB, FaceTime camera will start working again if I disconnect the Thunderbolt connection then reconnect it. If I have a cold start, the USB ports never work. It needs to be warm.

This holds true for an Macbook Air 11 inch 2015 laptop (Thunderbolt 2) and a Macbook Pro 13 inch 2017 laptop with the USB-C Thunderbolt 3 to Thunderbolt 2 adapter from Apple.