You started an emergency decryption on your computer, and pressed the space bar on the decryption screen. When you switched to the blue 'Progress %' screen, and pressed the space bar again, your computer stopped responding.
Sophos product and version
Sophos SafeGuard Disk Encryption
What to do
Restart your computer. The decryption process will start again once your computer has started up.
This is a known issue.