Luckily I did have another copy! I will now further remember to save multiple copies! lol

It could very well possibly be that last time you saved the file, the process was cut short for whatever reason and didn't finish re-writing the new file (updating/ saving as). That's happened to me before. I would have background programs running that might crash during a saving process of a map, and at that instant Windows Explorer would crash and restart meaning it would stop my Hammer in the middle of saving.

-Lesson: Keep multiple copies of your saves before re-writing the latest. I haven't done it yet, but a simple batch saving command could help.

I don't know what to look for, but open the .vmf in notepad and look through the entity list. Depending on how familiar you were with the map, you can tell if the list has stopped short. If it has, then that would mean it's incomplete and you probably lost some data.