The problem with fprintf

[jessie9a]

I write some codes below
FILE * fp;
int j=0;
char FileName[100]={0};

sprintf(FileName,"C:\\mttemp.txt");

if((fp=fopen(FileName,"a+"))!=NULL)
{

j=fprintf(fp,"hello\n");
fclose(fp);
}

After execute it , var j is 6,and the new file C:\mttemp.txt was created,but there is nothing in the file C:\mttemp.txt.
What's wrong?