It's hard to tell why since you aren't showing us all of your code. However, reopening the file should be pretty straightforward:
#include <stdio.h>
int main(void)
{
FILE* fp = NULL;
char name[] = "somefile";
for (;;)
{
// do something
if ((fp = fopen(name, "w+")) == NULL)
break;
// do something with the file
fclose(fp);
// do something
}
return 0;
}