Your computer has probably been updated to Windows 10. In October 2017 Microsoft began rolling out Windows 10 Fall Creators update, release 1709. As with any update, there were some problems and some unintended consequences. One change was how the Windows memory manager worked, which caused a problem for older installations of ExtendSim such that it could hang up or crash on launch.

If you are using ExtendSim 7.0 to 9.2 released prior to November 16, 2017, get your fix file here.