[KevinH]

If you used Sigil's Checkpoint feature you may have a recent backup.

If not, the only possible way for Sigil to corrupt a file is for the power to fail during an actual flle save operation and even then Sigil uses an atomic write to a temp file then copies it over so losing something during a power failure is actually very very hard and impossible if you use file save as.

First make a copy of the file that won't open onto a usb key for safe keeping.

Look in your os tmp folder and search for a known internal epub file names there as the working copies of the epub files may still exist on disk and therefore can be salvaged. You may even find the actual epub there if the power failed when saving under a temporary name.

If no luck then try running a disk utility program to help fix any interrupted file writes and make your drive accessible. This should work if you use a journaled file system.

You can also try renaming a copy of the epub file to be .zip and try a zip repair program on it since an epub is just a special zip archive.

Hope something here helps.