A couple of things come to mind. . .
I've had this issue come up before for me. I completely shut my computer down and re-started it. I'm guessing there were a couple of things going on that caused some internal com port conflicts (I'm not technically savvy enough to say for sure). After doing that, I got the computer and scanner to talk with each other.
Also, are you using Windows 8? Sometimes folks upgrading there operating system have found com port issues.
Lastly, if you can get to your Device manager, does it show the com port you are trying to use?