Corrupt save? Check this out:

This also happened to me, I'm working on a video to help people resolve it. It should be up on YouTube this weekend once I get back home.

But until then, here is what I found:

Googling launch commands for verify saved game brought me to the bethesda thread (linked below). Adding this launch option to the steam launcher seemed to work to ignore the dev mode messages:

+savegame_ignoreUnverified "1"

Saved games are stored under

%userprofile%\Saved Games\id Software\DOOM\base
or
C:\Users\(Name of the User)\Saved Games\id Software\DOOM\base

Here is how I got my save working again. Note: This may not apply to your specific issue, it just happen to work with mine.

1. Make a backup of your corrupt save, just in case.

2. Start a new game with a different profile (if you were using profile 1, start a new game with profile 2).

3. Get to the first checkpoint and exit Doom.

4. View the differences between what files each save game has. For example, my game was missing "game.details". My corrupt save had all other files intact.

5. Copy whatever files are missing in the corrupt save from the fresh save. (For me, I just copied the game.details file.)

6. Open up doom again and check out your campaign. It should kick you into developer mode if you didn't use the launch option. This is OK, as long as you don't care about achievements. You will need to exit and relaunch in order to use multiplayer though.

7. CHOOSING CONTINUE CAMPAIGN MAY OVERWRITE THE SAVE AND LOSE YOUR PROGRESS. MAKE SURE YOU HAVE A BACKUP OF THE FILES BEFORE ATTEMPTING. Press continue to see where it takes you.

8. Two things could happen after pressing continue:

• If that brings you to the beginning, just exit before the first checkpoint and go to mission select (or any of the other selections if you want to check their status, i.e. Collectibles). Select your last mission and start from there. You may need to exit Doom and copy your backup files back, overwriting the new games files that were created by hitting continue.

• If your lucky, selecting 'continue' will bring you to an empty room, or even better, your last checkpoint. I was stuck in some sort of dev-room, all weapons unlocked, no mods or map. Restart mission brought me back to where I left off when my PC crashed. I didn't try last checkpoint, which will theoretically work if you were mid-level.

I continued where I left off, with all my progress. Good luck.

EDIT: Clarifications and some articles:
Another possible fix someone posted to Reddit
GameFaqs thread about the corrupt saves
Bethesda Forums thread
IndiaToday brought the issue up