This type of error is normally caused by:
1. An object (not stock) that FPSC cannot deal with or find a file related to.
2. Something that has been removed or otherwise deleted from the game or program folders.
If it was working yesterday, that means that yesterday the program could find the file, and today it can't. If you are sure that you have not made any changes or deleted anything, then I don't have a clue what the problem could be.
Best.
The forum provides a means to hide our true identity. Too bad we always screw up and let it be seen.