When a GEDCOM will not open, it is nearly always one of a short list of things. Work down this list in order; most people stop at the first or second item.
A GEDCOM is a plain text file. Every check below involves looking at it. Any text editor works (Notepad, TextEdit, or a programmer's editor). If GED-VIEW can load the file at all, the raw GEDCOM Editor shows the same lines with numbers, which is easier for finding a specific line.
1. Is it actually a GEDCOM?
Look at the file name. GEDCOMs end in .ged (occasionally .gedcom). If it ends in .rmgc, .rmtree, .ftm, .ftb, .fdb, .gramps, .paf or something else, it is a program's own database, not a GEDCOM. Open it in that program and export a GEDCOM. The export guides cover each one.
If it ends in .zip or .gdz, it is a compressed package. Unzip it and look for the .ged inside.
If it ends in .ged but the first line does not read 0 HEAD, someone renamed something. Look at the contents and work out what it really is.
2. Is the file complete?
Scroll to the very end. The last line of a valid GEDCOM is 0 TRLR. If it is missing, the file was cut off, usually by an interrupted download or a full disk. Re-export or re-download. If that is impossible, adding a line 0 TRLR at the end will often let it load, with whatever was lost staying lost.
3. Is the header sane?
The first few lines should look roughly like:
0 HEAD
1 SOUR RootsMagic
2 VERS 9.0
1 GEDC
2 VERS 5.5.1
1 CHAR UTF-8
If the first line reads 0 HEAD with three odd characters in front, that is a byte order mark. GED-VIEW handles it; some programs do not. See the corruption fixes guide.
If there is no GEDC block at all, some tools refuse it. Adding the three lines (GEDC, VERS, and a CHAR that matches the file) usually fixes it.
4. Is the encoding what the header claims?
If names come out as Müller or with question marks in boxes, the file was written in one encoding and labelled as another. It opens, but wrongly. The garbled characters guide covers this.
If the file will not open at all and the header says CHAR ANSEL, it is an older file in an encoding many programs no longer read. GED-VIEW does.
5. Is it simply enormous?
Files with a few hundred thousand lines open fine. Files with millions of lines take a while, and some viewers give up. If GED-VIEW seems to hang on a very large file, give it a minute. There is a guide on why files get large.
6. Is it damaged in the middle?
Rarer, but it happens: a stray blank line, a line with no level number, a record that never ends. GED-VIEW is tolerant of most of these and reports what it skipped. If it reports errors, the line numbers point at the problem, and the corruption fixes guide covers the common ones.
Still stuck?
Use the support form under Help and say what program made the file, roughly how big it is, and what happens when you try to open it. Please do not attach the file itself; describe the first ten lines instead.