When you write a ROOT-file in a method with:

file->Write()

and the file exists, but is empty, check the right order of the files: Do

TFile* inputFile = TFile::Open("myInputFile.root");
TFile* outputFile = new TFile("myOutputFile.root", "RECREATE");
// your code
outputFile->Write();

but don't

TFile* outputFile = new TFile("myOutputFile.root", "RECREATE");
TFile* inputFile = TFile::Open("myInputFile.root");
// your code
outputFile->Write();

as your file will be empty.