Use Task Manager and see if it isn't still running in the background, stuck in perpetuity in the Task List. "Kill" the process, if it is. Try to run again. If it is NOT stuck there, then Windows might have "lost" what to do with the exe. Find your S3D.exe file (C:\Program Files (x86) \skwas \Silent 3Ditor folder), as well as the folder structure there. Try to start it directly from the S3D.exe file. Failing that, un-install the app again, and use a registry cleaner to remove "skwas" and "S3D" and "Silent 3Ditor" references, then re-install. I have had similar happen to me multiple times. Someone a while ago went directly to the issue... if I could only find that post... The main problem it seems, is that it is an older program, written to the WinXP paradigm...
Hah! it was from the swasjer himself!
a summary of things you could look at
You might also look to see if maybe MS, in their infinite 'wisdom', did an update on your machine, and wiped-out the netFramework v2, or maybe DirectX v9.0c on your machine. If the game runs, DirectX is OK. To check the framework, you want to "Add/Remove Windows Components", and see if it is selected in the list.