The supplied context only says that Ryland Grace "wakes with amnesia aboard the Hail Mary." It does not include any explanation for why he has amnesia.
If you want the canonical cause, please either:
- Specify whether you mean the novel or the (unreleased) film adaptation, and
- Upload or paste the relevant excerpt or chapter that describes how his memory was lost.
Once you provide that text or clarify the source, I can summarize the cause from the supplied material.
Sources
Tap to read source excerpt