The only two hopes you have are:
1. You have a backup file somewhere.
2. You have "Always create a backup copy" turned on in the
Tools, Options, Save tab. You could open the backup copy
which will have the same filename but have a .bak extension.
Otherwise, once a file is saved the previous version is
overwritten and is unrecoverable (except for paying the
really big bucks for a data recovery service.)