I found this on the steam forums and found it very helpful-
Have you joined a lobby and killed one person and that one kill give you the experience needed to reach level 70? What about the other way around, have you joined a lobby killed one person and gott all your experience taken away? resetting you to level one?
If you have then you are probably looking for away to get your level back. I'm writing this post sadly to tell you that you cant get it back, but! After you read this post you will know how to protect your self from ever having this happen again!
Your Mw2 profile is saved two places. On your computer and on the Steam Cloud. The files on your computer override the files on the Steam Cloud. So this allows you to backup the files on your computer and restore your level.
~How to do this~
Create a folder on your desktop and name it "Mw2 Profile backup" and open it.
Open up your steam folder "\Program Files\Steam" for you Windows 32 bit users and "\Program Files (x86)\Steam" for us 64 bit users.
Right click the "userdata" fold and select Copy, and now paste it in the "Mw2 Profile backup" folder.
Navigate your way to "\steamapps\common\call of duty modern warfare 2" in your steam folder.
Right click the folder named "players" and select Copy, and now paste it in the "Mw2 Profile backup" folder.
After Step 5 your profile is now backed up. (keep reading more important info at the end of this post.)
Now on to how to restore your profile once you have ran in to one of these *hacked* lobbys
Open up the "Mw2 Profile backup" on your desktop and make sure Steam is turned on and you are logged in. DO NOT open Modern Warfare 2.
Open up your steam folder as detailed above.
Copy the "userdata" file from the "Mw2 Profile backup" folder and paste it in your steam folder. When the prompt window pops up asking you if you want to replace select yes. Keep selecting yes until the process is complete.
Bet you cant guess what we do here... Open up "\steamapps\common\call of duty modern warfare 2" in your steam folder and then Copy over the "players" file from the "Mw2 Profile backup" folder and paste it in the "\steamapps\common\call of duty modern warfare 2" folder and again when the prompt windows pop up say yes to replace and keep selecting yes.
NOW! you aren't done yet. You still need to log in to Mw2 and play a game before it will be permanent.
I don't think this should be what it takes to make sure our data is safe but Steam isn't doing any thing to help us. Infinity Ward isn't doing any thing to help us and each time I've tweeted Robert Bowling (Infinity Wards Creative Strategist) I've been ignored. So what do we do? Do what PC users have always done. Do it our self!.
Now lets get on with what I really think. Every one of you PC gamers needs to create a Twitter account and send Creative Strategist Bowling your thoughts.
I'm not telling you to flame him. I'm not telling you to scream and cry to him. I'm telling you that he needs to hear our problems. He needs to listen. The only way that will happen is if we all force our way though the cloud of tweets he gets
His twitter account is @fourzerotwo. Again don't flame him and don't make a fool of your self making the rest of us PC users look like fools.