Hi MillieMayz and welcome.
If you will look in your FPSCreator directory, you will see a file named "userdetails.ini"
Here is what the contents of that file will look like:
[PERSONAL DATA]
YourName=
YourSerialCode=
YourFullPath=C:\Program Files\The Game Creators\FPS Creator
[LOCALIZATION]
language=english
Your file is probably missing the Name and SerialCode as in the one above. Add that info to the file, save it and restart the program. All should be back to normal. It's unclear why this happens, but it does more for some than others. You can make a backup of the userdetails.ini file in case it happens again.
Brian.