As an example, suppose I have

var
  f: text;
  filename, data: string;

Then the correct way to access the file is

begin
{...}
  assign(f, filename);
  reset(f);
  while not eof(f) do
    begin
      readln(f, data);
      {...}
      writeln(data);
      {...}
    end;
{...}
end;

where {...} represents any other processing or user interaction you want to
do. It sounds as if you are just using readln(f), or something like that,
which can't work because you don't provide a variable for the data read to
go into.

In addition, you do not describe your system at all.  You should, since
you post here, be using Turbo Pascal 1..7 or BP7; but we get questions
from users of other Pascals, Delphi, and even other languages.  Assuming
TP/BP, then it could be useful to know whether you're running under
plain DOS, or in a Win 3/95/98/NT DOS box, or whatever.  Are you
compiling for DOS, DPMI, or Windows (if you use TP, you may not know,
but it will be DOS)?

You do not say whether you have run-time checks on.  Borland distribute
Pascal with checks off; a programmer who is beginning to learn will put
them on & keep them on.  Alt-O C ...  a learner who has checks off will
be wasting his time and that of everybody he asks.  IMHO, something
about that ought to be in the mFAQ and other FAQs. My Web page
http://www.merlyn.demon.co.uk/pas-norm.htm#Checking refers.

You say the program breaks at "read in the file name" - presumably the
first time this is done.  Therefore all subsequent-in-execution parts of
the program are irrelevant to the problem; in a temporary copy of your
program, get rid of them, and anything earlier that is unrelated to file
name read.  If it now works, you've removed a misunderstanding; seek it.
If the error remains, it will be much easier for you - or us - to see.

If you have the least doubt as to exactly where the "break" occurs, you
may be able to use the de{*word*81}; single-step with (IIRC) F7.

Also, it's easy to add write statements - I tend to add Write(' *1* ');
Write(' *2* '); etc., but anything short and recognisable will do.