Launch an SSH client such as PuTTY or use SSH on the command-line. First connect to login.msi.umn.edu. From there, SSH to the NX node your session was running on. There are only two NX nodes: "noddy10" and "tern9". If you don't know which one your session is on, just try both.
For example, from login.msi.umn.edu:
$ ssh noddy10
Once you are are on noddy10:
$ pgrep -l -u $USER
You should see a number of processes. If not, exit and try "ssh tern9" instead, then try the ps command there.
You can try killing individual processes to see if it can unstick the session:
$ kill -TERM <PID>
or to kill all of your processes immediately:
$ pkill -9 -u $USER
Unfortunately it's difficult to determine which specific process might be causing the session to hang, though browser-related processes are usually a good guess.