It means the result "pointer" is invalid. Do you do any error checking on your database connection or result? Most likely your database is down.
$h=odbc_connect($db,$user,$pwd);
if (!$h) { die "Database connection failed!"; }
$sql = "...your query here ...";
$r=odbc_exec($h,$sql);
if (!$r) { die "Query failed: $sql"; }
while (odbc_fetch_row($r)) {
....your code here...
}