Think of ports as being like phones. If a phone is unplugged, your call will not be recieved. So you can call all you want, and you'll continue to get no response. If the phone is plugged in, then it will recieve your call. What it does with that call (talk to someone, VM, call forwarding, fax, etc.) depends on what service it offers. If the phone is on, but there is a block on certain numbers calling in or it possibly only allows outbound calls and not incoming calls, that is like a firewall. I could go on with this analogy, but I think you get the point.
So being able to open a port would like asking someone to plug in the phone that uses the number you happen to be calling. Or in your case, if you manage to get a command prompt, then plug in the phone yourself... start a service. Either start one that the system already has on it but not running or upload one of your own like netcat in listening mode. Now your phone is on and ready to provide a service.
On the other hand, if there is a firewall in place, then you may have to contend with that, too.
Hope this helps,
PS - Welcome to EH-Net 0cyberkill0.
CISSP, MCSE, CSTA, Security+ SME