4K USB C monitor worked one day and stopped working the next. Works in other OS.

<< Apple TB2 cable to TB2-TB3 adatper >>

That Apple adapter does not provide a usable display on the far side for any display protocol except genuine ThunderBolt.

About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Support

you need a different setup.

There are over 120 computers that bear the name "MacBook Pro" issued form 2006 to the present day. Which is yours, by year, early mid late, screen size and processor speed?

What version of MacOS are you using to connect?

EXACTLY what cables and adapters are in use to make the connection?

Early 2015 MacBook Pro are all 13-in built-in display and feature Intel Integrated Iris 6100 graphics, no NVIDIA nothing.

I presume you mean the Caldigit TS3 Dock.

You should be using the USB-C on the rear, marked with the ThunderBolt symbol, not the port on the front that is only labeled "USB 5Gbps" in letters.

Caldigit specs say this dock requires macOS 10.12 Sierra or later.

¿are you using a high quality cable for the connection to the display itself, such as the ThunderBolt cable that shipped with the Caldigit dock?

I don't know of any other display (that purports to respond to Industry-standard graphical interfaces) that has stopped working in a similar way.

Yes, there are displays that are problematic, even difficult. But stops working?

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as for fiddling around with display parameters to get it to work, SwichResX is the main player in that space. It lets you change away from the parameters the display directly provides (which is what MacOS Drivers use) and let you manipulate things differently, even arbitrarily. But exactly What to change in those parameters to is a mystery to me.

By contrast, you make arbitrary settings on Windows and the display adapter does what you specify. Regardless of whether that damages your display.

A frequent issue is [√] Mirror Displays. This setting checked forces the built-in and the external to use a common compromise resolution and timing. If one or the other can not handle the "compromise" settings, it can drop out.

Try un-checking that box and see if you can change resolutions to obtain a coherent picture.

The shortcut to toggle mirror displays on non-touchbar is command-F1 or command-fn-F1 -- on trouchbar it is command-Brightness-down.

<< Even the manufacturer said that Apple dropped the software for it. >>

got a link for that ?

Regardless, it may be time to shop for a better-supported Dock.