Unable to connect!!!

Hello im here cause i have no clue on how to fix this or even if their is a way to fix it....

This is my problem...

While i am runnin my server and after a few people log on....
its says in my CMD (which runs the server)///

unable to startup listener on 43594 - port already in use

this means i have to restart my server just to let people log on
i mean the ones already logged in are fine but if you log out and try to log in it will not let you
and people try to log in for the first time it will not let you if you know how to fix this then i will make you admin and do anything for you thank you....