Although you have started the program as usual, wait in vain for the synapsen main window to open. Nothing happens, not even an error message? - The reason is: the window has opened, but not in the visible part of the screen, for whatever reason. In these, however, you can easily direct the program in again by doing the following:
Finish synapsen if it is still active somewhere in the background.
Open a terminal/command line (cmd.exe) and type (depending on the operating system):
java -jar /Applications/synapsen.app/Contents/Java/debugsynapsen.jar -center
Windows or Linux
cd <IhrPfadZumArbeitsverzeichnisVonSynapsen> java -jar JSynapsen.jar -center
You will then see the program restart and take some tests. Then it ends automatically. Now everything is back to normal and you can start the program as usual.