.exe has triggered a break point - c

Question

I am very new to coding and I'm trying to learn how a compiler thinks.
Anyway, I wrote this code for school where the code is supposed to gather random data from an input.txt file, sort by insertion and then write the sorted version to an output.txt file. Also, the data needs to be 64-bits. I have two problems with the code. One is that during the function that writes it says '.exe has triggered a break point'. I don't understand why this is.
The second is that it says I haven't defined the variable key, even though I think I did. Below is my code. Any help would be appreciated as I'm very new to this.
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
typedef uint64_t u64;
struct file_contents
{
u64 *Data;
u64 Size;
};
file_contents
ReadFile(const char* FileName)
{
file_contents Contents = {};
FILE* File = fopen(FileName, "rb");
if (File)
{
fseek(File, 0, SEEK_END);
Contents.Size = ftell(File);
fseek(File, 0, SEEK_SET);
Contents.Data = (u64*)malloc(Contents.Size);
if (Contents.Data)
{
fread(Contents.Data, 1, Contents.Size, File);
}
fclose(File);
}
return Contents;
}
u64*
SortFile(file_contents Contents)
{
u64 key;
for (int i = 1; i < 100; ++i)
{
Contents.Data[i] = key;
int j;
j = i - 1;
while (i >= 0 && Contents.Data[j] > key)
{
j = j - 1;
key = Contents.Data[j + 1];
}
}
return Contents.Data;
}
void
WriteFile(const char* FileName, file_contents Contents)
{
FILE* File = fopen(FileName, "w");
if (File)
{
if (Contents.Data)
{
fwrite(Contents.Data, 1, Contents.Size, File);
}
fclose(File);
}
}
void
CloseFile(file_contents Contents)
{
if (Contents.Data)
{
free(Contents.Data);
Contents.Data = 0;
Contents.Size = 0;
}
}
void
WriteSortedFile(const char* InputFileName, const char* OutputFileName)
{
file_contents File = ReadFile(InputFileName);
SortFile(File);
WriteFile(OutputFileName, File);
CloseFile(File);
}
int main()
{
file_contents Contents = ReadFile("input.txt");
WriteSortedFile("input.txt", "output.txt");
}