Description of the problem

The client is listed in rrControl.

But it is shown grey (not connected).

You have restarted the machine but it does still not help.

How to find the issue

Application Log Files

The first place you should look are the Application Log Files.

The rrClient application log files are located here:  
RR/sub/log/C_....txt
This way you can check if the client starts at all.

Scroll down to the bottom and check the date of the last log entries.

Is there any information in the log from the current date?

Scroll up a bit and search for a log entry starting wirh WRN or ERR.

Background service log

If your rrClient is installed as background service 
and the application log does not contain any recent message (not even after you restarted the machine), 

then you should take a look if the service has logged an error.

The service error log file has one of these names depending on your OS and RR version:

Windows:

C:\Program Files\rrService\rrAutostartservice_Error.txt

C:\Program Files\RoyalRender\rrAutostartservice_Error.txt

Linux & OSX:

/usr/local/rrService/rrAutostartservice_Error.txt

/usr/local/RoyalRender/rrAutostartservice_Error.txt