Where have you been saving the files? We’ve seen similar reports in which it turns out that the users have been working with files saved directly to DropBox or in some cases on USB memory sticks. The previous save operation didn’t complete properly resulting in corrupted files. In the case of saving directly to a cloud location, connection time has been shown to be a problem. In some cases where the user was using USB memory, they’ve pulled the stick before the save operation has completed.
In rare cases SketchUp team members have been able to recover the file but more often than not it’s a lost cause. You could look for the SKB file which would be the last backup or if SketchUp crashed when the problem occurred there may be a recovered file. One of them might be usable.
Were there any “events” that you know of that might have caused the file to be corrupted?