The problem is actually rather simple, you work for hours, days, even weeks on your 'masterpiece' only to find one day that it won't open.

"WTF?? There goes all that hard work, down the drain!!"
Well.., not quite...

I'm going to show you how to get it back with as little fuss as is possible.

First, lets navigate to where your session files are stored.
In a default install of Steam, you can find your session files here:

The actual session files are saved as .dmx files, as seen below:

The .dmx files are the files that SFM opens when you open a session.
You will also notice an .autosave file with the same name as the .dmx file.

Next, we will rename the .dmx file to add '_old' to the name.

There's a reason for this and you will see it here shortly...

Now we rename the .autosave file to be a .dmx file.

You will most likely get a popup asking you if you really want to do this.., yes, you really do want to do this.
DO NOT PANIC!
The very next time that SFM autosaves your session, a new .autosave file will be created.

Now, open SFM and load your session as normal.
It should open and be useable. If it is, you can now go back and delete the '_old.dmx' as it is corrupt and not useable anyway.

You may have to redo some small things that you already did and I will explain why.
The .autosave file is like a backup of your session. It contains most, if not all, of the info on your session and is created and updated automatically by SFM usually every 5 minutes, by default.

Question: What if I don't have an .autosave file?
Most likely your SFM's autosave function has been disabled. You will need to re-enable it.
Why, in God's name, anyone would intentionally disable autosave is beyond me, but regardless, some folks do, and that nullifies this fix.
If you don't have an .autosave file, or a backup copy of the original .dmx file, then you are basically screwed. Sorry, but that's just the facts.
The only way that I know of to enable/disable autosave is through the startup commandline.
Here's more info on that: https://developer.valvesoftware.com/wiki/SFM/Command-line_startup_options

I have heard that you can open a .dmx file in a text editor like Notepad++ and fix it that way, but a quick Google search didn't turn up a whole lot of info on exactly how to do this without making matters even worse. So, if you are familiar with coding, you are welcome to try this method out, but I know almost nothing about this process so I won't even pretend to recommend it in this guide.

As always, if you have any questions or comments, please feel free to post them below and I will get back to you as soon as possible.