I would say this is not a literary prompt, in summary it would mean open your hidden and buried secret. That s what the open coffin would be emphasizing. And truly, the figurative expression can be literal in some cases, take for example, someone who buries his secret in the coffin. If he were to figuratively open his coffin, then he will open the coffin he buried for real. It's a good expression, I like it.
The participation link:
@mariannewest/day-1978-5-minute-freewrite-thursday-prompt-open-your-coffin