This error occurs where: a) for some reason the simulator, HFRACR.EXE, has been left open in the background after completing a simulation, or b) for some reason the operating system is not allowing this program to open. Try a couple of things to assist us (assuming the simulations worked previously on this computer):- Try rebooting the system, or check with task Manager, to insure the HFRACR is still open some way,- If the problem persists, please use Windows Explorer to navigate to the folder where StimPlan.EXE is installed. In that folder there is another file, HFRACR.EXE, execute that directly with  a double-click and see if the problem still occurs that the simulator will not open.