You will need to contact WeChat to see if they can restore those chat logs for you.
To contact WeChat..
WeChat ID: WeChat
Add "WeChat Team" on WeChat to contact them and send a message
When did you send a message?
Please click here: http://www.wechat.com/en/feedback.html
Fill out the form and let them know if the issue through that route as well
Have you heard back yet?