1) Open Regedit from the start menu. Go to 'HKEY_LOCAL_MACHINE/SOFTWARE/WOW6432Node/Activision/Call Of Duty 4'
2) Look for the 'codkey' value. 

(I strangely did not have this value so I created it (You want to create 'string value' if doing this) and entered in the cod4 cd key I have entered ingame. If you do this, this should fix the issue and your stats should be back to normal.)

3) If you see the 'codkey' value Right click on it and select modify. You should see that the cd key listed isnt the same as the one you have ingame.
4) Open your cod4 and replace the value in 'codkey' with your ingame cd key. Close and reopen cod4. You should now find that your stats are back.