MDB_INVALID: File is not an LMDB file

[Chris Trenkamp <chris@christrenkamp.com>]

My program suddenly started returning this error, "MDB_INVALID: File
is not an LMDB file" whenever it tries to open the database file.  It
is running on Windows.

I made a copy of the database, compiled lmdb with MDB_DEBUG set to 2
and ran this program:

#include <stdio.h>
#include "lmdb.h"

int main(int argc, char* argv[]) {
  int rc;
  MDB_env *env;
  rc = mdb_env_create(&env);
  printf("%d\n", rc);
  rc = mdb_env_open(env, "./brokedb", 0, 0644);
  printf("%d\n", rc);
}

And it gave this output:
0
mdb_env_read_header:4040 page 0 not a meta page
-30793

Does anyone know how this might have happened, and things to look for
in my program that might have caused this?  Is there a way to recover
the data from the file?

Thanks.