Model A1407 is not a 27" LED Cinema Display. It is a 27" Thunderbolt Display.
The 27" LED Cinema Display (model A1316) requires Mini DisplayPort input. You would connect it to a modern Mac using a USB-C (DP) to Mini DisplayPort adapter or cable. You would not use an Apple TB 3-to-2 adapter, because that adapter will not present an unwrapped DisplayPort signal on the TB2 side.
With the Thunderbolt Display (model A1407), it's the other way around. It needs Thunderbolt input. So you would use the Apple TB3-to-2 adapter, in connection with a Thunderbolt 1/2 cable. A USB-C to Mini DisplayPort adapter would not carry a Thunderbolt signal from your computer to the display, and so would not work.
You say that this display was working, but no longer works.
- Is there any chance that you accidentally swapped out a Thunderbolt connection for a Mini DisplayPort one? Should be easy enough to fix by swapping back.
- Is there any possibility that the display's attached Thunderbolt cable has become faulty? If it has, you may be able to work around that. Get a Thunderbolt 1/2 cable and run it from the adapter into the display's TB daisy-chaining port.