Thunderbolt monitor goes dark
This is a known issue with that era of MacBookPros. You may be eligible for a the MacBook Pro Repair Extension Program: If you are, they will fix it for free even if you are out of warrantee (I had my mid 2012 repaired under this program.)

Apple has determined that a small percentage of MacBook Pro systems may exhibit distorted video, no video, or unexpected system restarts. These MacBook Pro systems were sold between February 2011 and December 2013.

https://www.apple.com/support/macbookpro-videoissues/