If you load a file name without any path information, it will only be found if it is located in the current directory - normally the same folder as the EXE file, but its doesn't have to be as several things can change it, including a shortcut to the EXE file in Windows which allows the user to specify the "Start in" folder.
And in production, storing data which changes in the same folder as the EXE file is a no-no as it's generally write protected for virus security.
What you need to do is store data in a "generic" folder that is available (but different) on every PC. Have a look here: Where should I store my data?
] - it gives code for locating data under user folders.