For an Apple Thunderbolt Display (27-inch), model A1407, which has two heads on its "hydra" cable, you want one of these: Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter . If the permanently-attached cable turns out to be bad or flaky, you could try bypassing it with a Thunderbolt 1/2 cable running from the TB2 side of the adapter to the TB daisy-chaining port on the display.
For the very similar LED Cinema Display (27-inch), model A1316 – which has three heads on the "hydra" cable, you need a USB-C to Mini DisplayPort (female) adapter.
Be sure that you have correctly identified the display. If you use the wrong adapter, the plugs will physically fit, but you won't be giving the display the signal type (Thunderbolt or DisplayPort) that it expects, and you will get no picture.
In the case of a LED Cinema Display, you would probably also want to connect the USB uplink in some way.