There could be several reasons:
1. The simulation takes a lot of time to complete (the CPU is high and a Model.exe file is consuming CPU in TaskManager)
    (by the way, you can kill the .exe to return the control to SimForge).
2. The simulation goes into an infinite loop due to some bugs in simulation runtime.
3. It could be a problem in SimForge (but I doubt this).

If you can't provide a (public) model that has this behavior we  can't help you too much.
However, you can always try a nightly-build of OpenModelica:
  http://build.openmodelica.org/omc/builds/windows/
I assume you have Windows, but I might be wrong, so you use Linux.
Note that we haven't tested the nightly-builds with SimForge so they might not work nicely together.

You could also try to use a different solver (the default is method="dassl"):
simulate(model, method="dassl2") or "euler"

Cheers,
Adrian Pop/