Hello kaan48964,

To begin with I would recommend that you try to verify the integrity of the game cache:

1. Start Steam and right click on DayZ in your game library (make sure that the game is not running).
2. Select the tab labeled "Local Files".
3. Click on the button labeled "Verify integrity of game cache...".
4. Start DayZ once the process is complete.

You can also try the following:
Delete all battleye folders in your app folder "C:users\<username>\appdata\local\dayz\battleye" and in your steam folder "C:\Program Files (x86)\Steam\steamapps\common\DayZ\BattlEye". Then create new battleye folders there and copy the battleye.dll which you can download from battleye.com: http://battleye.com/download.html

If this does not solve the problem, here's a suggestion that is a bit more drastic. Please be aware though; you might have to reinstall your other Steam games afterwards:

1. Move the entire "C:\Program Files (x86)\Steam\SteamApps" folder to your desktop. Your "SteamApps" folder should now be at the following location: "C:\Users\<username>\Desktop\SteamApps".
2. Move the "DayZ" folder out of the "C:\Users\<username>\Desktop\SteamApps\common" folder and subsequently delete the "DayZ" folder.
3. Uninstall Steam while making sure the "SteamApps" folder is still on your desktop.
4. Install a new version of Steam.
5. Go to the new "C:\Program Files (x86)\Steam\SteamApps" folder.
6. Move everything inside the old "SteamApps" folder on your desktop to the new "C:\Program Files (x86)\Steam\SteamApps" folder.
7. Install DayZ again.

Regards,
JStewart