If an agent becomes disconnected for whatever reason, you can follow these steps to re-connect it:
ssh to the server that the disconnected agent is on.
go to the build agent’s bin directory (i.e. /opt/teamcity/buildagent/buildagent01/bin/)
su to the user that the agent runs as (i.e. appBuilder)
run agent.sh start
That will start the agent, and after a few seconds or so, the next time you go to TeamCity, it will be connected.