Some One copied my itunes backup folder from my pc, How can I keep my data secure now

Why didn't you locked your PC user account?

The backup content without your Apple ID & password cannot be used.

Next time, do an encrypted backup, but if you forget the encrypted backup password, you will not be able to restore from it.

He will not be able to use the content you purchased from Apple.

Other data can be extracted, examples are your messages.