Atleast im not the only one, was proper annoyed when this happened, i enjoyed unlocking things, testing them as i got them. Only bought the game recently, actually quite enjoying it but things like this are a downer.
When it happened the first time i googled deleting the profile, there is a bunch of folders in the Steam folder with numbers for a name (i.e. 34829) These house your profile (they are not distinguishable, and may affect other games). I removed them and it kicked me back down to level 1, in hindsight i should have backed them up before i played as it happened again).
I'll try and find the process
Edit: Found googling, not the same method i used but seems ok
"21. Vista / Windows 7: How to get multiplayer rank back after crash ?
If your running Windows Vista or Windows 7 and have not backed up your players directory you may still be able to recover it.
1. Browse to -- C:|Program Files|Steam|steamapps|common|call of duty modern warfare 2 or for 64bit Windows -- C:|Program Files (x86)Steam|steamapps|common|call of duty modern warfare 2
2. Right Click on the "players" directory and click properties.
3. Click the "Previous Versions" tab
4. If you have previous versions listed then your in luck.
5. Copy the "players" folder to "Players - Old" so that you have a backup.
6. Delete the contents of the existing "players" folder and copy the files from previous version back in to the dir.
7. Restart MW2.
22. Technical support says about Back-Up :
1. Go to the Steam folder and find the Modern Warfare 2 install folder
2. Default location: C: |Program Files | Steam | Steamapps | common | call of duty modern warfare 2 |
3. Open the | players | folder
4. The multiplayer profile is updated regularly as a. Stat file and also includes a backup file.
5. All original profile files begin with md202ef8d and backup files will begin with md202ef8d.bak
6. Locate the profile and backup. Stat files with the most recent "Date Modified" listed in Windows
7. Move the profile. Stat file to a new location (Creating a new folder and placing it here should be fine)
8. Rename the backup. Stat file and remove the ". Bak" from the filename
9. Launch Steam and re-test this game.
"