First of all, in the Outputs Pane, go over the run that is running, and do Right Mouse->Output Log and see if the simulator is running and what it is doing - maybe the simulation has stagnated because of an unusual corner condition causing convergence difficulties?
If that doesn't help, then in the Run Summary assistant (see the attached picture), there will be (at least one) terminal window for each running "ICRP" process. You can click the Right Mouse button over this and get a popup of the job log - near the top you'll see info such as this:
\# Initial memory used: 36659200 (0x22f6000) bytes
\# process size: 374894592 (0x16587000) bytes
\# Qt version: 4.5.3
\# Window Manager: other
\# User Name: dummyUser
\o Working Directory: dummyHost:/export/home/dummyUser/demos/OCEAN
\# Process Id: 18231
From this you can see the process id - and you can kill this process id (from a UNIX terminal, using the kill command). Maybe there is some reason why the ICRP has got stuck (maybe it says down the bottom of the log file). Generally if the ICRP is killed, it will restart it and try again.