You have likely seen this problem with other software, as have we.
Snappy fax, like many other programs, is designed so that only one instance can be running at any time. When snappy fax starts, it looks to see if another instance is already running. If so, it sends a message to that instance to make itself visible and then the starting instance exits.
The already-running instance doesn’t respond to the message and show the main window. This is because it is in an unstable state for some reason and cannot receive and process the windows message.
To fix this, use Task Manager and click on the ‘Details” tab. Scroll down and find sf5.exe. Right click on it and select “End Task” from the popup menu. This will kill the already-running instance so you can now start a new instance of it.