One of your builds fails to run, and stays in the build queue, as shown in this image:

Inspecting the Agent Requirements page for the build configuration will reveal the following:

TeamCity shows you that a previously compatible build agent is now incompatible.

Cause

From TeamCity plugin version 3.0.0 onwards, SQL CI is no longer included in the plugin. This means that if any your build agents didn't have DLM Automation installed and you upgraded the plugin, those build agents will no longer be compatible.

Fix

To solve this, you should install DLM Automation on all required build agents.