Mixbus auto-saves a "backup" of your session occasionally. Hopefully you can find a backup that recovers most or all of your recent changes. Here's how to recover a backup snapshot:
Open the session folder
Open the "backup" subfolder
Find the latest date-stamped snapshot
Drag the snapshot file up one level to the main session folder
Launch Mixbus, and you should see a triangle next to the session's name.
Click the triangle to show the snapshots, including the backup snapshot.
Double-click the snapshot to open it.
You can then do a "snapshot->and switch" to rename the session to something better, and continue working on it.