Accessory Thunderbolt ports on Thunderbolt monitor not working
There are 5 different things you can try to to reset your TB Display


Make sure you have the latest macOS update.  Click on the Apple logo in the upper right corner of your screen.  Select "About this Mac" then click on "Software Update."
Power Cycle for 30 seconds.  Unplug your all of your peripherials and Mac from the TB Display and then unplug it from AC power for at least 30 seconds.  Reconnect your devices starting with your Mac.
Reset your SMC(Differs by Model)
Reset your NVRAM.  Starting with your Mac off with AC adapter plugged in, hold down CommandOptionPR while turning on your Mac.  Hold it until it chimes 3 times at which point, release the keys.
Try a standalone TB cable.  Use just a plain TB cable to connect your Mac to your TB Display and/or your peripherals.  If things start to work then your dual Mag Safe cable may be bad.


If all of the above fail, then you probably need to take it in for service as the TB Display logic board has failed.

Source:  MacRoumors.com