'E.T.' Screenwriter Melissa Mathison: Will Missing ... Fortune on the Line

Melissa Mathison -- the 'E.T.' screenwriter and ex-wife of Harrison Ford -- is at the center of a mystery ... her will is missing, and turns out a lot is on the line because she's worth a bloody fortune.

Melissa -- who died last month from cancer -- had written a will but new court docs reveal her business manager can't find the original.

The court docs show Melissa is worth $22 million. Her fortune is all in a trust, but the document was not included in the legal papers so we don't know who's supposed to get it. She had 2 kids so it's a good assumption they'll get at least part of it.

So now the question ... what happens if the original will is never found? In California, as long as no one's fighting over her estate, a copy will do just fine.