Hi,
I'm very sorry to hear about the 3123 error. This error is a very low level error from the underlying database Muse uses. It typically indicates the file has been corrupted by something outside of Muse (i.e. failed file transfer, failed file synchronization, file on media that was removed without being ejected, etc.),
Was the file stored in a synchronized folder (i.e. Dropbox, Creative Cloud Connection, other) or on a server?
It would be helpful if you could send us the .muse file at muse-support@adobe.com along with a link to this thread. (If the file is larger than 20Mb you can use a service like Adobe SendNow, SendThisFile or WeTransfer.)
It's very rare that a file that reports this error can be repaired, but if possible we'll repair the file. Even if it's not possible, examining the file may help us determine the underlying cause of the corruption.