what are the ways a null pointer is returned when i try to open a file inthe following line

FILE *Fptr;
Fptr=fopen("try.txt","r");

I'm using turbo C++ 3 compiler (its an old box)
and its on a windows system
its unable to the file though its in the same folder etc and also this only happens some times not always it does manage to open the file some times why does this happen

I am using the Cpp compiler for C

thanx in advance