This means the agent is 1) not running 2) hung / hosed / otherwise not working
I would restart your agent as first step.
This can happen if agent settings are not correct. I would also make sure you are running SC 3.5 some issues were fixed that caused agent problems. SC 3.5 also adds agent logging which provides more details about why something may have not worked. Agent logging is enabled in config and is on by default. Agent logs are locaged under logs/<profile> on agent.
Please check you should have settings like this:
In addition if this continues to be problem you can run without agent. Just install SC server on maxdb host directly and leave SC_AGENT blank.